That has to be first and foremost on your list of concerns. Tubs should be covered when not in use, and provision made to keep small children away from open tubs.
Hot tubs are hot.
Small children have much less ability to withstand heat stress than older children, adolescents and adults. I personally would advise you follow a rule that until kids are out of diapers, they are too small for the hot tub. Again, to me this is a good reason to keep little kids out of the tub. Germs like warm water. Proper chlorination of hot tubs is essential.