Snorkel package from Costo about $50. West Bay Beach is shallow, clear, and like glass most days. Your daughter can see fish waist deep, she doesn't have to be in 15' of water, that's a silly statement by that poster.Yes use flippers. Buy a child's snorkel vest, your daughter NEEDS to have one of these. 1 puff of air in the vest and she will be buoyant and you won't have to worry. She will become much more confident with it on. It is not like a life vest it adjusts with very little air to give you the correct buoyancy. If you need it to be a life vest a couple of puffs and you will float upright.
You can see all you want to see in the shallows of the reef on West Bay. Child's snorkel vest online about $20-25. Remember it is an evolving process, what may be a new experience the first day will quickly change to a normal activity. This is the time to start, as I said before the first hour our son had issues with all the gear, but get past that and your daughter will be dragging you out to the reef.
And once she has become a fish you will be out in the deeper water which on West Bay is only 8-10'. the water over the reef is crystal clear, You don't have a sense of depth that would scare her. Only when you are in blue water is when you have a sense of real depth, but there is no coral to be seen snorkeling from that area. You would never need to be deeper than you can stand up in at first.
When our son had a snorkel vest on WE were much more comfortable and as long as he was within arms length to grab him we were ok. He spent hours at a time on the reef when he was 4 after we got past the initial break-in period which is a little frustrating for everyone.
If you want I can email a picture of him on West Bay with all his gear on. This may help you to show your daughter whats involved and that other kids her age have no problem with it. As I said our son now has been diving since he was 10 and got his open water certification last summer so there really is a good reason to introduce them to snorkeling early.