I want to call my mum mummy, I want to speak more childish, and be more childish
Bit dodgy around your actual Mum - why not try and find a local ABDL scene or some friends to hang with/chat to? There might be more scope for that there.
Also, it's waaaaay better to do with a boy/girlfriend, so... Maybe save it for that moment, too?
I can't see a way it'd ever be OK to act like that without initiating worry about your behaviour, sorry bud!