Why the baby calls Hiro - papa and Masaki only Maa-chaan?

Maybe so he doesn't get confused if they made him call both of them papa?
or because Masaki is basically the Mom but he's too embarrassed to be called Mama?
tbh idk either, but it's still cute that Masaki is called Maa-chaan

'chan' at the end of a name (in this case maa) it is somewhat of an honorific but only used when you're really close to someone and it's supposed to be adding like a 'cute' aspect to it