"The baby sits on my lap."

Translation: 아이가 제 무릎에 앉아.

[EdwardDuck20]

How is 아이 accepted, but not 아기, if the translation is "baby"?

[Komerinam]

If you use 제, it should have 앉아요. And when you use 앉아, they should add 내 so it can sound more natural.