Are you running the spa uncovered during this time? If not you need to. It can sometimes take 24 to 48hours for the bromine to drop in a new tub because there is no bioload for the bromine to react with and oxidize.
test only free chlorine and multiply by 2.25 to get total bromine
10 to 15 ppm is ok but don't rush it. It really takes a few days to a few weeks to get everything stabilized. When you do put in the floater you will need to adjust it until it maintains your desired bromine level based on your tub usage. Be patient. It often takes a few months of 'tweaking' until everything falls into place.
Water chemistry adjustments don't happen instantaneously.
Children do not dissipate heat as well as adults so turn the temp down to around 95 - 98 and limit him to about 5 or 10 minutes at a time. Keeping hydrated is very important.
https://hottubinsider.com/hot-tubs-safe-kids/
https://www.verywellfamily.com/when-can-grandchildren-hot-tub-spa-1696192
https://www.bullfrogspas.com/blog/hot-tub-safety-kids/