We were there a few years ago when my sons were 4 and 5 and both went in rainbow reef. Near the entrance there is a shallower area where they could swim but stand up if needed. Neither one were brave enough to leave that area for the deeper part. We were there at a really quiet time of year and the cast members were really helpful with the kids. Surprisingly a lot of fish still came in the shallower area. I recommend a length of stay pass, that way there's no pressure on your child to try and figure it all out in one go. Our boys would go in for 10 min at a time and then go warm up (rainbow reef is freezing when you're used to the swimming pool temps!) and go back several times a day. I will say, don't have high expectations for a 5 year old. We live on the beach and our kids are in and out of the water constantly. Still at age 7 and 8 snorkelling can be a tricky thing for them. Helps to get a mask before you go and let your child play with it in the bathtub to get used to the feel. Also if your child takes a mask down to the beach at Aulani try renting a boogie board too- here at home our kids learned first by laying on a boogie board (while you're holding it) and looking down in the water with a mask. They're more relaxed when they're not trying to swim as well. Good luck!