Step-parent problems

[Mini C]

My ex-husband has had a new partner since the breakdown of the marriage and my children stay over every few weeks for a couple of nights. The girlfriend is young and has no children of her own but insists on bathing and dressing mine calling it 'girl time'. On the last visit at 'girl time' they cried wanting their mommy and she smacked them. They did not tell their father but now who do I tell, in normal circumstances I would not let my children near anyone that smacks them.