Interestingly, there appears to have been an alleged assault on a NorthWest flight recently (see Rumours and News).
Agreed that most assaults are carried out by someone known to the child (not sure where you got the idea it was the mother, Squawk - everything I've read has suggested male family friends and stepfathers figure more prominently in statistics). However, I guess, as FA's, your job is ultimately passenger safety and comfort, and for UM's, that involves more than just giving them colouring in pencils and a coke. You are there to cater for the needs of the passengers, whatever their age. Therefore, in order to fulfil your duty of care, if that involves moving passengers, then so be it, unless you can think of another way of managing it?
I don't think it's fair for males to be thought of as guilty before proven innocent by the way. Perhaps a by-product of an overly litigious society?