Hi,
yes, indeed PlayMaker already feature actions such as GameObjectHasChildren, GetChildCount, GameObjectIsChildOf
If you want a gameobject to be aware of a reparenting, it's not possible until Unity 5.3. I just updated the PlayMaker Utils on the Ecosystem with a new component called "TransformEventsBridge", so get the latest PlayMaker Utils and use that to send events when ever the parent or children changed. Warning, this is only available int Unity 5.3 or newer.
Let me know how it goes.
Bye,
Jean