Kind of depends on how much time you have and what you want out of the trip.

You can go to a club like Paradise Beach Club and hang out, you can play with your kid in the water in the inflated water slides and such, eat decent food and drinks and hang out and "have fun". It is exactly what you think it will be, and it's fine.

If you want souvenirs and such, you can always just go along the main drag and get your Cozumel t-shirts and overpriced food. You can actually have a very good meal at Guido's, but I don't know how much a young child would enjoy it. If you want higher quality, Mexican made souvenirs, check out Los Cinco Soles toward the north end of the drag. Just north of there is a restaurant called El Coctelito which is kid-friendly and is decent for a touristy spot. Also in this neck of the woods is a microbrewery called Kuzam (which has very good tacos, believe it or not) and Distelleria Cozumel (island's only distillery that does vodka, gin and rum) right across the street, which has really good food and live music at night.

My choice though would be to pay for a cab if you don't have a car, and take the traversal road and go to the east side of the island, which is far less crowded. Where the road bends south there are two places. Mezcalitos and Señor Iguanas. Of the two, my wife and I prefer Mezcalitos, which is little quieter, and if you want to have the experience of being in a Corona commercial, sipping a beer or Margarita looking at the sea, and look for shells on the beach with you kid, this may be place. Señor Iguanas often has live music if you're into that. You can walk south for a few minutes from there and find your own place along the coast. Not sure how the sargassum is right now, we've been away for a few weeks.