Well, you have a point, since this is One Piece world so lol at living costs.
Still, just trying to think about it logistically, you have housing, food for multiple people (especially with how big mom eats), doctor check ups (you need to keep them healthy after all), basic home needs. Education if they have that in OP.
I would understand it more if she sold the other children the marines don't buy into slavery or something, but the chapter implies she just routinely has a whole orphanages for which she only actually uses one kid with huge intervals inbetween.
It just doesn't seem financially sound, or it wouldn't if this wasn't OP. She should be selling way more orphans, is my point.