Why do you think "Freak" pretended to be Robert Man in day care?
I think this was Kevin's (Freak) tongue-in-cheek creation to signify that because his legs were supported by braces, he was more like a robot than a boy. So, instead of pretending to be a robot, he was pretending to be a boy (or man) instead. No doubt it is Freak the Mighty author Rodman Philbrick's play on words to reference the old science fiction automaton, Robby the Robot, who first appeared in the film, Forbidden Planet (as well as other sci fi flicks). Robby became one of filmdom's most famous robots. Robby the Robot--Robert (the) Man. Get it?