Folding paper should be possible even with the limited motor control. I imagine the first few cranes are all crumpled, but you could get REALLY bored just sitting around being useless. Boredom triumphs over handicaps.
@13man18 - they all can do that. Hina is just lucky she got her hands on some paper, because other babies don't have any paper and can't show off their skills. Being a useless baby weighs down on them, and they forget their previous lives as a result.
So THAT'S why her brother is so listless when he is older! lol
@Kaarme I would argue that babies don't know they need to get stronger, and thus don't bother to try (duh, they're infants) but she is clearly conscious at this point (and for who knows how long before this scene?) so she could have been purposely trying to be independent as quickly as she could.
I actually JUST read an isekai where the MC was an infant and doing muscle training to stand on her own.