We have decided to have kids at our wedding, right now it looks like there is going to be 5 of them in total. I am trying to include all of them in as much as I can and then for the adult only events (i.e. the stagette etc.) we have already talked to some of the grandparents about babysitting. To try and include them, I have made them their own welcome bags with beach items in it, all five of them will be in the wedding party and then have a blanket set off to the side with some activities so during the ceremony they can occupy themselves and finally I am making up "Cancun wedding adventure" coloring books for during the dinner. All five of them are generally very well behaved plus the oldest is going to be almost 9 and he has agreed to keep an eye on all of them throughout the wedding day. As for the rest of the trip our resort does have a kids club and lots of kids activities so parents can relax as well.
My fiance and I looked at doing an adult only however we didn't want to risk not having our family and friends come down just because they couldn't find someone to watch the kids.