As far as bringing snorkel gear it's a pain in the #$$ to bring on a plane. It's heavy and bulky. However my experience has been that "rental gear" is good for a few minutes if you're lucky. After that the leaks
, breakages and poor fit make snorkeling unpleasant.
I'd suggest hunting round and trying a few types on. I prefer the medium length full foot floating fins as one can gently put a foot on a bit of coral
or even nudge a sea urchin without ruining the holiday. Face masks should be checked for fit by seeing if they held onto the face without the strap by sucking through the nose. Basic snorkels are more reliable than the fancy ones. If you do get a fancy one bring a backup.
For me, trying on stuff in a rental shop is one of life's less enjoyable experiences, and then it has to be returned...
Don't forget sun protection shirts for wearing in the water
and childrens' blockout cream.
My limited experience of life jackets has been that children are very difficult to fit. I'd suggest taking them into the store and "guiding" their selection. One with a crotch strap is going to be better than one without, but a good fit should be the first priority. I've found that life jackets are remarkably easy to pack!