It truly depends.
If it is during the dry season (hubby and oldest son went in May) then it would not be dangerous. Only you know if periods of "dark" would scare your child.
However, I would not recommend it during the rainy season. Those caves have get swift.
I would recommend an independent tour only because they link the floaters together. On Carnival tours...you are on your own.
If it is the dry season, being on your own means you do all the work (and it is work to paddle through low water). With Yhony, they pull you. In May, there were even some Carnival people trying to grab onto Yhony's group...which of course, Yhony's floaters told them NO WAY.
If it is the rainy season, then the caves get swift and if you are on your own...you can easily go somewhere you don't want to go. Again, an independent tour has you linked so you only go where you want to go!