My husband & I have a hot tub & a 23 month old. I thought it would probably be okay to have our daughter in the hot tub with us if we kept the temp at about 97-98. I asked the advice of my brother that is a doctor at the Mayo Clinic. He made the following comments:
Never at a public pool or hot tub regardless of temp due to bacteria & chemicals
Not advisable at home (it's NOT like a bathtub) Chemicals are harsh. It can cause breathing problems due to jets and bubbles. Risks associated with filtration suction, bacteria and tender skin of a child under 48 months.
Bottom line, my brother's concerns went way beyond water temperature, but more about the chemicals, bubbles and fine mist, causing quality of air problems. His advice to us was even if we addressed those issues, to wait until our daughter was above 4 years old and limit the time to 10-15 minutes @ 97-98 degrees.
He said don't think of a hot tub as just a large bathtub, it's not..