I'm creating quests. As an hunt task, there's no way to select a BabyMob, the mod always resets to Zombie

Found out that adding them manually works.

I know this is a pretty old issue by now but baby mods are defined by a single NBT tag. They aren't separate entities from the adults and that's why there's no distinction between them in the selection screen. Just select the adult you want and toggle isBaby in the tags to get the desired baby subtype.