How have you managed to resize a container - doesn't it resize itself automatically to fit all its children? Even if you have overridden some method to allow manual resize, a child node probably won't detach automatically if the container is resized to no longer contains the child geometrically. Perhaps you should handle the NodeModified event raised for the container, and loop over its SubordinateGroup.AttachedNodes to call Remove for the ones that are no longer inside - i.e. when container.Bounds.Contains(child.Bounds) returns false.

I hope that helps,
Stoyan