Going to Cozumel and only shore diving is a little like going to Disney World and spending your time on a swing set at the hotel. Swing sets are fun, for sure, but there's no comparison to what's inside the gates at Disney World. There's a vast difference between shore diving in Cozumel and diving the reefs from a boat. Vast.
If you hire a private DM, you'll be set. You don't need to get a private boat, just hire a private DM. It generally costs about $50 extra and would be worth every penny to you and your child. The private DM can work with both of you to make sure you're comfortable diving from the boat, and that you and your child have safe, fun dives.
If you want to glide into it, maybe you could do both. The first day, do a shore dive or two with a guide/DM and then the second day go out on a boat with a private DM. It's an extra $100, but money very well spent. If you arrange it through one shop, you would very likely have the same DM for both days.
I've never been diving with them or stayed at their hotel, but Blue Angel sounds like a good fit for you this trip.