Hi,
If you have PlayMaker Utils in your project, there is a component "TransformEventsBridge" which will listen to hierarchy changes on your GameObject, it allows you to fire an event, and so use this to check only when required how many child you have and take appropriate steps then.
I also created a new custom action on the Ecosystem GetChildCount2 which will expose an everyframe option, though, it's not optimal and I suggest you use the component above.
Let me know how it goes.
Bye,
Jean