That is how I taught my kids to snorkel. They ride your back, I had them more forward and they look over your shoulder.. and we stayed on the surface. If they start choking or having a problem, they will instantly and instictively push you down as they lift themselves above water. This is a very noticeable response/feeling and you can spin around and grab them and deal with any issues within 1-2 seconds. It provides excellent safety and control and they get a lot of confidence.
It is much easier and safer than holding their hand or just watching them, because that does not provide instant and constant feedback on their status. It doesn't take that many sessions of "back riding" before they bail off and want to dive on their own.
I had my twin 4 yr olds snorkeling over shipwrecks in 90 feet, it sounds crazy to me now, but they liked it. Too bad it was pre-GoPro.
If they have been snokeling and diving down and clearing ears for 4-5 years in the Ocean, it is not too hard to teach them to scuba when they are 10.