I think hitting the children on the head with books etc is not acceptable, regardless of what they were/weren't doing. Poor kids. The fact that dad felt it was totally acceptable to do this in front of strangers and at Church suggests that he sees no wrong in doing so. What's scarier to wonder is what would he do at home behind closed doors?
Did the mum tell dad off at all?
I think a comment to let him know that he didn't need to hit the kids would have been warranted. If the dad hears from other people that they don't think his behaviour is socially acceptable, then he may be forced to think about it. If everybody just turns a blind eye, then he's getting the message that what he's doing is ok. It's a hard one, b/c you obviously don't want to get involved in conflict (or get hit over the head with a book, yourself!).