I assume that the sensitivity you're potentially concerned about is in the ballpark of minor irritation, not a major allergic reaction, right? Is there any reason you couldn't go get some springtails from whatever source you'd get them from and have the kid with sensitive skin handle them and some associated dirt to check for a reaction?

In any case, I would think that even if the bugs themselves have some possibility of causing irritation with lots of direct contact, you'd have to be doing a whole lot of prolonged and messy digging in the dirt to experience it. Maybe you'd have messy contact like that while doing the initial setup, but the whole point is that after that you mostly leave it alone. You don't go digging it up all over the place all the time beyond removing the biggest chunks of solid waste.
I could be totally off here, but it seems to me that it would take an pretty potent irritant for it to fill the soil to the point where contact with a snake living on it would cause irritation.

But in any case, springtails are incredibly widespread. There are many species and they basically live everywhere there is soil or anything like soil. I would hazard a guess that it would be much easier to cause an infestation in the house by tracking dirt in on people's shoes than because you had some in a bioactive tank. And that assumes that there is a hospitable environment for them in the house in the first place.

Basically, it seems like even the worst case scenario for springtails being potentially irritating could be avoided by wearing gloves to go digging in the dirt and vacuuming up any that spilled on the floor, which you would presumably do anyway.