(I also thought of surgeonfish as a possibility, not freshwater of course, but that's just asking for someone to get a nasty cut eventually, and they are also pretty susceptible to disease)
I've seen surgeonfishes in touch pools before, not with the purpose of being touched though. They stayed to the deeper parts and just avoided hands.
Rainbow bass maybe but they could mistake children's hands for food.
If you mean rainbow trout, I've seen them as a feed only attraction before. (As in toss trout pellets in the pool, not hand feeding) They go bonkers for food and put on a pretty good show.