We stopped in Biloxi on our way to Florida in July. We stayed at Beau Rivage and the kids (ages 4-10) thought it was great despite the fact that the pool was closed that day due to bad weather. There's a big arcade area that should keep the kids occupied for an hour or two. And I know first hand that you can lose a little money in a short amount of time at the casino there.
Another family option would be Margaritaville, which has all sorts of things to do but I don't think it has gambling (looked into staying there but didn't actually go there).
I'm not sure how big your group will be but one good thing about Beau is that you can get connecting rooms. When I called Margaritaville, I was told that it wasn't an option - not sure if that's because they were all booked or they just don't have connecting rooms. But we have four young kids, so connecting rooms is a must in the rare occasion when we stay at a hotel instead of a VRBO/AirBnB.