Personally, the next time there's a visit, I'd give the little boy a heads up (nicely) and say something like "play nicely, no hitting etc. otherwise I'll take my kids home." And if it does happen, then take them home immediately.

I think being prepared is the best thing so that you know in advance what you're going to say and can then do it calmly. It's a toughie on how to handle the mum as it sounds like you do want to be friends with her, but just imagine if the situation was reversed and it was your kids hitting her kids ... I'm sure you wouldn't blame her if she took her kids home.

I wouldn't offer the mum advice unless she specifically asked for it. But I would maybe try to convey that by taking your kids home, you hope you're doing her a favour and supporting her in trying to get her kids not to do that sort of thing. Maybe something like "oh, it's awful when they're going through that phase isn't it ... hopefully if I take the kids home he'll get the message that it's not on like you've been telling him."

Ahhhhhh ... another parenting minefield.