What are you on about? You don't seriously think anybody would say 'Humphrey this is for your mum, Irene this is for your mum, and Hubert this is for your grown up'? I think you're looking to take offence.
I've been teaching long enough to know that this is entirely possible. The point is every single staff member at your school currently uses mum and dad. If a child started then it's entirely feasible that they would revert to their default unintentionally causing the poor kid to feel otehred. Either that or they would try hard to make a conscious effort to get it right and again make it obvious the child was different to their classmates.
Not looking for offence at all but it just seems beyond nonsensical that such a simple change of language is not already implemented in every school.