He said "おマメ扱いすんなよ！" which roughly translates to "Don't treat me like a child!".

マメ - "Mame" means bean. It was written in Katakana. it clues us into knowing that it's referring to the casual term for bean which is baby. :D

It's a bit of a rude way of saying it. Haha. I think Nino was kind of annoyed when he said it, that's why it came out that way.

Nino was adorable here too :P <3