Remove uses of TransformComponent.ChildEntities (#22442)

* Make output of ChildEnumerator non-nullable

* Remove uses of ChildEntities

* poke tests

---------

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/Temperature/Systems/TemperatureSystem.cs

@@ -341,7 +341,8 @@ public sealed class TemperatureSystem : EntitySystem
     {
         RecalculateAndApplyParentThresholds(root, temperatureQuery, transformQuery, tempThresholdsQuery);
 
-        foreach (var child in Transform(root).ChildEntities)
+        var enumerator = Transform(root).ChildEnumerator;
+        while (enumerator.MoveNext(out var child))
         {
             RecursiveThresholdUpdate(child, temperatureQuery, transformQuery, tempThresholdsQuery);
         }