One possibility you might consider is to a) take photos of them with your phone when this unacceptable behaviour is happening, or b) enlisting help from a close friend to take some photos of this behaviour. When you have enough photos to prove that this is not a one off situation, then I would start planning.
Try and arrange for a time when you are with the parents , but the children are either in bed or elsewhere. Then you can engineer the conversation to some of the behaviour, and ask in a calm way if they do the same things at home or are they allowed to behave like that at school playtimes. If the parents either genuinely dont know what you are on about, because the children do not behave like this elsewhere, you say oh look I have a couple of photos from last wednesday etc and show them. That way you are not speaking in an accusing or sort of tell tale way, but quite innocently showing what happened last time etc. Then you can take your lead from their reactions. Now I can see that this could cause problems between your husband and you, but at least the subject will have to be addressed, and this should lead to everyone knowing what exactly is happening and what needs to change.
I am with the others in that , if after you have had this discussion, the behaviour carries on, then every time I would collect your coat or whatever and leave. If you have one car, then I would also take that and leave without stating a time you will be back. This will mean he has to deal with the children, either get a taxi to take them home, or contact the parents and explain that you have gone out and they need to collect them. If that does not ring alarm bells with the parents , then I think there is a bigger problem that it now appears, and would be very concerned about the situation.
Whatever the outcome, he has shown poor judgement at the least in his treatment of you and of the children. He is as you say wanting to be mr nice guy, but he is not their age, although he is behaving as if he was, and the children will learn at some point in the future - preferably sooner rather than later - that their behaviour will gain them no friends and they are more likely to be seen as bullies and parents of other children may decide that they do not want them to be part of their childrens friends. So in the long term , he is actually making things much worse and his lack of adult judgement needs to be addressed. If he becomes very defensive when spoken to by either the parents or you, then it is something that really needs to be addressed for his sake too. But whatever happens , you are NOT wrong at all in your feelings and attitudes , and you have every right to refuse to be any part of this scenario. As an intelligent adult you are right to make sure the parents know what is happening and then it is their responsibility to deal with both the children and the father. Not only are you behaving as a responsible adult should do in these circumstance but you are clearly showing your husband that there are lines that he may not cross and get away with. I hope things can be resolved in a positive way but from what you have said , there is definitely a situation that needs to be dealt with and at the earliest possible time. You might mention to your husband that he might not enjoy this if he allowed it to go on until they were as strong as he is and he would come off worse!! If only selfish things matter to him the thought that he might get hurt could have some force!! Good luck and dont let him try to con you ,that you are in any way responsible for whatever happens next. Should he blame you that his behaviour is no longer tolerated without realizing that he has been behaving in a totally unacceptable way then I would simply state that in the same circumstances you would speak up in the same way.
I do hope that things can improve, but whatever happens you are correct in seeing it as something that must be dealt with.
