Adults have a more balanced thermostat and even after spending ten or fifteen minutes in a temperature of 105 degrees, can still maintain an appropriate body temperature that allows all the bodily functions to work properly. However, a child does not have this capability and thus it is not safe to take an infant into a spa. It's also not particularly appealing to the other adults who will be using the spa to have a child in diapers share it with them!
I would also avoid taking a young child (three to six) into a spa, partly for the reasons mentioned above, and partly because of the potential for drowning. The central area of many of these spas is one to two feet deeper than the edge, and children may falsely think that it is not over their heads. Also, the suction that pulls the water out can be very strong, and there have been reports of children who have been pulled under the water by the force of the suction.