I know I'm not reader ,but Let me give you some advice.
How are you childish ? Like give me some Examples .Are you childish Because you like and childish things ? Cause if you are I see No problem with that ! I'm literally like that.
If you're childish in a bad way , like Whiny,(not implying that you are)etc. Then of course you need to change that. Oh and ask them to stop.
My advice is Ignore them my brother calls me *******(Not a curse word Just a Nickname) and it stinks ,but I've learned to ignore it and know When he says it it doesn't even phase me.