My daughter was cleaning her cat's litter box at that age, but under close supervision, both while scooping it out and making sure that she washed her hands well after - it was not a punishment, just one of her responsibilities as a pet owner. She wanted the cat, she had to take care of it, including the icky bits.
BTW, according to my vet, toxoplas is pretty rare, and cats pick it up by eating infected birds and mice. So i f you have a strictly indoor kitty, there's no risk of toxoplas. The top concern regarding litter boxes and mouths is E.coli.
As for potty mouth, I actually didn't have an issue with her using "special" words, as long as they were at home, AND as long as she used them correctly. She knew that there were times and places where certain words simply were not allowed - school, for example.
I would NEVER put soap in a child's mouth.