Infants and toddlers should not be permitted in a hot tub at all as babies' thin skin makes them more susceptible to overheating. Also, since little ones have very little control over bodily functions, spas become unsanitary almost instantly when "accidents" happen.
No young child should be allowed in a hot tub until they can stand on the bottom and have their head remain completely out of the water. Children who are big enough to be in a hot tub should not use it for more than five minutes at a time, especially at the maximum temperature of 104 degrees. Dropping the spa temperature to 98 degrees would allow for longer soaks – but never more than 15 minutes at a time. It is also recommended that young children avoid full body immersion, choosing instead to sit on "jump seats" that some spas have that permit waist-high immersion.
All persons, and especially young children, should be encouraged to drink fresh water while they're soaking. If the bather feels sick to their stomach, dizzy and / or sleepy, they should exit the spa immediately.
Responsible Adult Supervision is Key to Child Pool, Hot Tub and Water Safety. There should always be an adult designated to maintain constant visual contact with children whenever they are near, or could get near, any body of water.