Me: Mono, Two girls, 12 and 9
Partner: Poly, Older son (20), daughter (12)
Metamour: No kids
Everyone is "out" with respect to the kids and the kids are doing well with it. My kids know that he has another girlfriend, and after some "huh?" questions ("Wait, you mean EX-girlfriend, right?"), it's no big deal. I don't treat it as one. My girls and his daughter get along swimmingly, and his son is cool with it all - doesn't understand it (hell, *I* don't understand it), but sees his dad happy and that's what matters to him.
We do not live together (or even really all that close to each other), so we don't all get together that often, but where the kids' schedules intersect, we try to get them spending time with each other. They have a great time when they do.