We were there in June last year and it was very hot. We did most of our exploring in the morning and evening and enjoyed being in/near the water and the blessed ac for naps. I can't comment on the places you name as we rented a small apartment for the few days we were there. You might consider use of a kitchen a priority since it gives you added flexibility for meals, especially nice with a toddler in tow. Happy trails!

Have stayed at Maria del Mar five times, always in May. Stayed in the cabana rooms which are by the pool, in the garden area, nice and quiet, these rooms (especially the ground floor ones) would be good for a toddler. They all have AC and small fridge and two beds. Maid service and grounds keepers are very good. Only minor drawback is the shallow end of the swimming pool is a little deep, might have to help your child get in and out depending on age. They also have the tower rooms closer to the beach and the castle rooms which are newer (above the restaurant across the street).

Have only eaten at the restaurant and had drinks at Na Balam so cannot comment on the rooms, sorry.

Yes, May is hot, but the ocean is cool as is the pool. We get out early to explore and back to the condo for lunch and lounge by the pool until cocktail hour, then off to watch the sunset and dinner in town w/shopping. Check out Ixchel Condos/Hotel. I've not stayed at Maria del Mar or Nabalam but IMO from the outside looks, Ixchel is much nicer and newer, great beach-front, pool w/restaurant. Have Fun!

I think Maria's would be better with a child, what with the pool. Na Balaam tends to be more of an older adult crowd. Both have equal access to north beach. You can still use the restaurant and beach bar and enjoy the grounds (which really aren't that extensive) at Na Balaam even if you don't stay there. There's probably more trees and thus shade directly outside of Maria's than there is at Na Balaam. Plus, Maria's is a tad closer to downtown. It will be hot in May, but stake out a good spot early under a palm tree and you'll be fine. Take one day and spend it at Playa Lonchero and enjoy a great fish lunch right on the beach. One more thing, the tower does tend to be noisy since its right over the kitchen. Isla is great, even in May, and Maria's is the best value on the island, IMO.