It depends on the social/cultural situation of the character, how they grew up, their personalities, their parents' personalities... lots of things.
My character Ingvar grew up as a prince. I imagine it would make sense for him to address his parents in a more formal, proper way - so he calls them Mother and Father (or he did, before his mother passed away). Or 'sir' sometimes, in the case of his father.
Koen and Fay are growing up with a loving but poor family - they love it though, despite the fact their father is always gone. So they use the more affectionate, informal terms of 'Mum' and 'Da' (now that they're older; it used to be 'Mummy', but it was always 'Da' for their father).
So yeah; for me, it depends on the situation.
