I'm not an expert on children's snorkling abilities...but here's what I can tell you...
There are two snorkel "routes"...one is a shorter route, for beginners...and the other is longer route. If your 5-yr is a very strong swimmer and likes to snorkel (and is with an adult)...then it should be fine. If the thought of fish brushing up against his/her feet is going to freak him/her out...then I'd think twice about it.
My hubby and I have seen a number of kids w/adults who do fine...then other kids get freaked out about not seeing the bottom (it's the ocean, not a pool...makes a difference to a lot of folks...not just kids )....or having "live" fish swimming with them, etc....
Hope this helps! Let me know if I can provide more info.
My now 7 year old, made it 3/4 of the way to the end to the snorkeling when he was 5 but I had to swim him back because he was just too tired. Last week he made it all the way and back! As the previous poster said it depends on your 5 year olds ability, we have a swimming pool so he is and was a very strong swimmer. There are no "set" routes so you can take him out for as far and as long as he is comfortable. He will love it
My 6 then 7 year old did most of the trail also on two trips. The inflatable vest helps. Keep the vest flat and get out to where the water would be over their head. Then start off by inflating his or her vest and making the child just stay still, no swimming, no moving, so they can realize that they can rest upright and catch their breath or clear their mask this way. With good technique they do not need to expend a lot of energy to get out there. I did end up towing him all the way back the last time because he was freezing (January) and pooped. The first time (October a year ago) he put his hands on one of the bouys and was screeching from being stung by the nematocysts that live on them. We just inflated his vest and gave him time to relax and get over it. I did the same with my vest so I could hold him until he was O.K. The guard did volunteer to surf him in and my son was ready to bail, but once we got him calm and the sting subsided, he stayed for another hour. Just keep an eye on how far out you are, it can be a long way back with a "monkey on your back". But the guards will never let you get in trouble. Our 9 then 10 year old stayed out as long as all the adults.
Our 6 yo totally refused to snorkel the first time we put him in the ocean when he got swamped by a wave and came up spewing. The next day, we had paid for an expensive trip in a boat in 30-60 feet of water. He got into the water, snorkeled for over an hour and did not want to quit. You just never know. They will decide for you.