Just let people play whatever they want as long as it isn't trolly. At the end of the day this is a game and people want to have fun. If people can't do the roleplay they want, then they will leave. It's that simple.
I would prefer it not be done to be honest because I don't like it and to be frank find it odd that an adult would want to pretend to be child, but at the end of the day my personal beliefs are let people do what they want so long as it doesn't effect others negatively.