First of all, thanks for your reply. And yes, it seems to be correct, when checking in Scene hierarchy the child object is indeed moved under the parent object (and in order to double-check, I also used Get Parent and logged the result in console).
I'll give some details, maybe it helps(?)...
1. Both GOs are instances of prefabs. Naturally, I refer to these using PlayMaker global vars, to be able to manipulate them from any GOs FSM.
2. I use the Set Parent PM action to attach one GO to the other as child.
3. A third GO's FSM is rotating the parent GO in the scene, using iTween Rotate To action. And the child oesn't follow then. If I replace here the iTween action to Transform.Rotate then everything is fine.
I don't know but it seems to be purely iTween related problem.