I have to throw my opinion here, while Zimbali would be a great place for a meal, or to stay if it was you and your wife, I just have to say I think the kids would be straight up bored. Sorry...
I think they will love Xtabi, but I would rethink Zimbali and consider a place on the beach, maybe a place like CoCo La Palm, or Crystal Waters, that have Villas that would suite you, the beach scene would be such fun for your sons and fiancé with things like Volleyball at Alfreds, and Margaritaville,and lots of music, just my 2 cents, have a great time !