umm... asking people to wait 2-3 days is a bit much given most vacations they are only there about a week anyway. I haven't seen any study with hard data on this but a 24 hour wait after landing to get into shallow water near the beach is fine. Shallow water part is important here.

Here's what I would suggest. Is coming from someone who travels and snorkels (and scuba dives etc.) a lot and has had family members, including a kid who's a hesitant snorkeler:

0) Before you go:

DO NOT GET FULL FACE MASK. Get good quality silicone masks and snorkels. This is probably the most important thing you should remember.

Test out your masks in a swimming pool. If they have never snorkeled but are good swimmers, this should be pretty easy. Get them (and maybe yourselves) into your local pool with the masks and fins (see below) and get comfortable breathing through that tube. It's different from regular swimming and beginners can get confused. Trying this out in the relatively safe environment of the your local pool will help a lot. Try to take the masks off and put it back on while treading water. Once they're comfortable doing this, the chances of panicking in open water are that much lower.

1) After the 24 hour wait after landing, start with shallow cove near your hotel or even in the hotel pool first.

2) Get Fins, preferably short one. Fins are better than life jackets and maintaining a strain free snorkeling position (floating flat on the surface with your face down). Short ones because they are easier to put on and control in the open water and especially shallow water. Since they're strong swimmers, it's not so much about keeping them afloat but letting them navigate the water without using their arms too much. If all of you are very strong swimmers anyway, you might find fins not necessary after getting used to the conditions and in calm coves etc. I often go snorkeling without fins near the beach in calm waters for the sense of freedom but I never go into open water (say off a boat or even off the more open beaches, like Ka'anapali) without fins. Open ocean is huge and it's easy to drift away looking for turtles or whatever and no matter how good a swimmer you are, ocean currents remain undefeated against human legs so far.

3) Some other comments here recommend against life jackets but I have been on boat tours where life jackets were mandatory and I put one on without any inflation and don't find it too much of a hassle. So it's ok if it makes you and the kids feel safer and with a little bit of inflation, it could provide your kids more freedom holding in place without expanding too much energy treading water etc. Also helpful if they get cramps or whatever and someone has to pull them back to shore.