I have a 5 year old son who loves the hot tubs. However, he has been taught (by me) that he is to just SIT there, not splash, not swim, not going under, and certainly not jumping in. If a young child sits there (like any other adult would), I don't see a problem with him/her being there, with an adult of course. No child should ever be in a hot tub without their parent/guardian present. I do not allow him to stay in the hot tub for more than 5-10 minutes, either.
However, I DO take exception to babies. I don't care if they are not crying. Hot tubs are TOO hot for diaper-age children, PERIOD. Here is just one comment I found on the topic:
The hot temperature in a hot tub also poses danger to small children. A child's skin is much thinner and burns more quickly than an adult's skin. Since the skin of children is not able to regulate temperature as quickly as adults, it puts children at an increased risk for dehydration and overheating. Children have been severely scalded by hot water in a hot tub. It is usually recommended that children under the age of 5 not be allowed in hot tubs because of their sensitive skin and the potential safety risks of a hot tub for children that age. Children between 5 and 12 years of age must be taught safety rules when in a hot tub.
When are some parents going to wake up? When their baby is scalded???