diff --git a/src/ZB.MOM.WW.MxGateway.Server/Galaxy/GalaxyHierarchyIndex.cs b/src/ZB.MOM.WW.MxGateway.Server/Galaxy/GalaxyHierarchyIndex.cs
index 091cffb..5378a61 100644
--- a/src/ZB.MOM.WW.MxGateway.Server/Galaxy/GalaxyHierarchyIndex.cs
+++ b/src/ZB.MOM.WW.MxGateway.Server/Galaxy/GalaxyHierarchyIndex.cs
@@ -7,18 +7,21 @@ public sealed class GalaxyHierarchyIndex
     private GalaxyHierarchyIndex(
         IReadOnlyList<GalaxyObjectView> objectViews,
         IReadOnlyDictionary<int, GalaxyObjectView> objectViewsById,
-        IReadOnlyDictionary<string, GalaxyTagLookup> tagsByAddress)
+        IReadOnlyDictionary<string, GalaxyTagLookup> tagsByAddress,
+        IReadOnlyDictionary<int, IReadOnlyList<GalaxyObjectView>> childrenByParent)
     {
         ObjectViews = objectViews;
         ObjectViewsById = objectViewsById;
         TagsByAddress = tagsByAddress;
+        ChildrenByParent = childrenByParent;
     }
 
     /// <summary>Gets an empty Galaxy hierarchy index.</summary>
     public static GalaxyHierarchyIndex Empty { get; } = new(
         Array.Empty<GalaxyObjectView>(),
         new Dictionary<int, GalaxyObjectView>(),
-        new Dictionary<string, GalaxyTagLookup>(StringComparer.OrdinalIgnoreCase));
+        new Dictionary<string, GalaxyTagLookup>(StringComparer.OrdinalIgnoreCase),
+        new Dictionary<int, IReadOnlyList<GalaxyObjectView>>());
 
     /// <summary>Gets the object views.</summary>
     public IReadOnlyList<GalaxyObjectView> ObjectViews { get; }
@@ -29,6 +32,9 @@ public sealed class GalaxyHierarchyIndex
     /// <summary>Gets tags indexed by address.</summary>
     public IReadOnlyDictionary<string, GalaxyTagLookup> TagsByAddress { get; }
 
+    /// <summary>Gets direct children grouped by parent gobject id. Root objects (no parent, or self-parented) live under key 0. Each list is sorted areas-first, then by display name (OrdinalIgnoreCase).</summary>
+    public IReadOnlyDictionary<int, IReadOnlyList<GalaxyObjectView>> ChildrenByParent { get; }
+
     /// <summary>Builds a Galaxy hierarchy index from the given objects.</summary>
     /// <param name="objects">The Galaxy objects to index.</param>
     /// <returns>A new Galaxy hierarchy index.</returns>
@@ -71,10 +77,39 @@ public sealed class GalaxyHierarchyIndex
             }
         }
 
+        Dictionary<int, List<GalaxyObjectView>> childrenByParent = new();
+        foreach (GalaxyObjectView view in views)
+        {
+            int parentKey = view.Object.ParentGobjectId;
+            // Treat self-parented (corrupt) rows as roots; matches DashboardBrowseTreeBuilder.
+            if (parentKey == view.Object.GobjectId)
+            {
+                parentKey = 0;
+            }
+            if (!childrenByParent.TryGetValue(parentKey, out List<GalaxyObjectView>? bucket))
+            {
+                bucket = [];
+                childrenByParent[parentKey] = bucket;
+            }
+            bucket.Add(view);
+        }
+
+        foreach (List<GalaxyObjectView> bucket in childrenByParent.Values)
+        {
+            bucket.Sort(CompareByAreaThenDisplayName);
+        }
+
+        Dictionary<int, IReadOnlyList<GalaxyObjectView>> readOnlyChildren = new(childrenByParent.Count);
+        foreach (KeyValuePair<int, List<GalaxyObjectView>> kvp in childrenByParent)
+        {
+            readOnlyChildren[kvp.Key] = kvp.Value;
+        }
+
         return new GalaxyHierarchyIndex(
             views,
             viewsById,
-            tagsByAddress);
+            tagsByAddress,
+            readOnlyChildren);
     }
 
     private static string BuildContainedPath(
@@ -110,4 +145,27 @@ public sealed class GalaxyHierarchyIndex
 
         return obj.TagName;
     }
+
+    private static int CompareByAreaThenDisplayName(GalaxyObjectView left, GalaxyObjectView right)
+    {
+        if (left.Object.IsArea != right.Object.IsArea)
+        {
+            return left.Object.IsArea ? -1 : 1;
+        }
+        return string.Compare(DisplayNameOf(left), DisplayNameOf(right), StringComparison.OrdinalIgnoreCase);
+    }
+
+    private static string DisplayNameOf(GalaxyObjectView view)
+    {
+        GalaxyObject obj = view.Object;
+        if (!string.IsNullOrWhiteSpace(obj.BrowseName))
+        {
+            return obj.BrowseName;
+        }
+        if (!string.IsNullOrWhiteSpace(obj.ContainedName))
+        {
+            return obj.ContainedName;
+        }
+        return obj.TagName;
+    }
 }
diff --git a/src/ZB.MOM.WW.MxGateway.Tests/Galaxy/GalaxyHierarchyIndexTests.cs b/src/ZB.MOM.WW.MxGateway.Tests/Galaxy/GalaxyHierarchyIndexTests.cs
new file mode 100644
index 0000000..39f6c39
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Tests/Galaxy/GalaxyHierarchyIndexTests.cs
@@ -0,0 +1,96 @@
+using ZB.MOM.WW.MxGateway.Contracts.Proto.Galaxy;
+using ZB.MOM.WW.MxGateway.Server.Galaxy;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Galaxy;
+
+/// <summary>
+///     Coverage for <see cref="GalaxyHierarchyIndex.ChildrenByParent"/> — the parent→children
+///     index used by the lazy browse projector (Task 3). Verifies root grouping, nested
+///     parent→child linkage, corrupt self-parented row handling, and the areas-first
+///     ordering rule shared with <c>DashboardBrowseTreeBuilder</c>.
+/// </summary>
+public sealed class GalaxyHierarchyIndexTests
+{
+    /// <summary>Verifies roots (ParentGobjectId == 0) bucket under sentinel key 0.</summary>
+    [Fact]
+    public void ChildrenByParent_RootsUnderSentinelZero()
+    {
+        GalaxyObject root1 = new() { GobjectId = 1, ParentGobjectId = 0, ContainedName = "r1" };
+        GalaxyObject root2 = new() { GobjectId = 2, ParentGobjectId = 0, ContainedName = "r2" };
+        GalaxyObject root3 = new() { GobjectId = 3, ParentGobjectId = 0, ContainedName = "r3" };
+
+        GalaxyHierarchyIndex index = GalaxyHierarchyIndex.Build([root1, root2, root3]);
+
+        Assert.True(index.ChildrenByParent.TryGetValue(0, out IReadOnlyList<GalaxyObjectView>? roots));
+        Assert.NotNull(roots);
+        Assert.Equal(3, roots!.Count);
+        Assert.Contains(roots, view => view.Object.GobjectId == 1);
+        Assert.Contains(roots, view => view.Object.GobjectId == 2);
+        Assert.Contains(roots, view => view.Object.GobjectId == 3);
+    }
+
+    /// <summary>Verifies a nested A→B→C chain links each parent to its single child bucket.</summary>
+    [Fact]
+    public void ChildrenByParent_NestedHierarchy_LinksParentToChildren()
+    {
+        GalaxyObject areaA = new() { GobjectId = 1, ParentGobjectId = 0, IsArea = true, ContainedName = "A" };
+        GalaxyObject objB = new() { GobjectId = 2, ParentGobjectId = 1, ContainedName = "B" };
+        GalaxyObject objC = new() { GobjectId = 3, ParentGobjectId = 2, ContainedName = "C" };
+
+        GalaxyHierarchyIndex index = GalaxyHierarchyIndex.Build([areaA, objB, objC]);
+
+        Assert.True(index.ChildrenByParent.TryGetValue(0, out IReadOnlyList<GalaxyObjectView>? underRoot));
+        Assert.NotNull(underRoot);
+        Assert.Single(underRoot!);
+        Assert.Equal(1, underRoot![0].Object.GobjectId);
+
+        Assert.True(index.ChildrenByParent.TryGetValue(1, out IReadOnlyList<GalaxyObjectView>? underA));
+        Assert.NotNull(underA);
+        Assert.Single(underA!);
+        Assert.Equal(2, underA![0].Object.GobjectId);
+
+        Assert.True(index.ChildrenByParent.TryGetValue(2, out IReadOnlyList<GalaxyObjectView>? underB));
+        Assert.NotNull(underB);
+        Assert.Single(underB!);
+        Assert.Equal(3, underB![0].Object.GobjectId);
+    }
+
+    /// <summary>Verifies a self-parented (corrupt) row appears under root, not under itself.</summary>
+    [Fact]
+    public void ChildrenByParent_SelfParentedObject_AppearsAsRoot()
+    {
+        GalaxyObject selfParented = new() { GobjectId = 5, ParentGobjectId = 5, ContainedName = "loop" };
+
+        GalaxyHierarchyIndex index = GalaxyHierarchyIndex.Build([selfParented]);
+
+        Assert.True(index.ChildrenByParent.TryGetValue(0, out IReadOnlyList<GalaxyObjectView>? roots));
+        Assert.NotNull(roots);
+        Assert.Single(roots!);
+        Assert.Equal(5, roots![0].Object.GobjectId);
+
+        // The self-parented row must not appear as its own child — bucket either absent or empty.
+        if (index.ChildrenByParent.TryGetValue(5, out IReadOnlyList<GalaxyObjectView>? underSelf))
+        {
+            Assert.Empty(underSelf!);
+        }
+    }
+
+    /// <summary>Verifies children sort areas-first, then by display name (case-insensitive).</summary>
+    [Fact]
+    public void ChildrenByParent_SortsAreasFirstThenByDisplayName()
+    {
+        GalaxyObject parent = new() { GobjectId = 1, ParentGobjectId = 0, IsArea = true, ContainedName = "Root" };
+        GalaxyObject zebraObj = new() { GobjectId = 10, ParentGobjectId = 1, IsArea = false, ContainedName = "zebra" };
+        GalaxyObject alphaArea = new() { GobjectId = 11, ParentGobjectId = 1, IsArea = true, ContainedName = "alpha" };
+        GalaxyObject betaArea = new() { GobjectId = 12, ParentGobjectId = 1, IsArea = true, ContainedName = "beta" };
+
+        GalaxyHierarchyIndex index = GalaxyHierarchyIndex.Build([parent, zebraObj, alphaArea, betaArea]);
+
+        Assert.True(index.ChildrenByParent.TryGetValue(1, out IReadOnlyList<GalaxyObjectView>? children));
+        Assert.NotNull(children);
+        Assert.Equal(3, children!.Count);
+        Assert.Equal(11, children[0].Object.GobjectId);
+        Assert.Equal(12, children[1].Object.GobjectId);
+        Assert.Equal(10, children[2].Object.GobjectId);
+    }
+}