It's shocking that they were detained. It's also shocking that the father could have them released but hasn't done so. His children saw him hit their mum, it's understandable they might not want to see him, never mind be ordered to attend lunch with him.
I hope it's a one off and those children are returned to mum. But if it becomes a regular thing, it is a feminist issue, because I think violent and controlling exes or husbands will use it to threaten and control.