I'm pretty sure the surrogate parents are the ones who are biologically having the child but not raising them. That is, the surrogate mother is the person carrying the baby. They don't get paid leave (because they're not raising the kid) but they can get unpaid leave (presumably to give birth and rest before and afterwards). I would assume the parents would be taking care of the surrogate parent(s) as part of the arrangement though.
I don't think any of that is really transferable to poly.