It does sound like you shouldn't have these kids at your place anymore. I understand what some posters are saying about 'your home, your rules', but if no one else is going to back up you and your husband then it sounds like you would need to spend every minute policing these children. I like kids and I'm pretty lax on rules in my home ( shoes on the couch are fine with me ) but I wouldn't be able to be a good host, or a happy host, if I felt I was responsible for making sure two children didn't fall off a balcony or into a pool.
I would probably skirt the edge of honestly if asked for reasons - 'Oh, we just feel like our place is too small for two active children', or something like that. That way, if pressed, you could try again to get some support from your in-laws. "We can try hosting at our place again, but we will really need your help making sure the kids stay safe and out of trouble".