Most Helpful Guy
It depends. If he has the kid (s) all the time (meaning their father is not in the picture or there is no babysitting support from other family members), probably not, since kids get sick, injured, etc., plus I honestly don't want someone else's around during all my free time. Another complication is that I have kids myself, so everyone would have to get along (too many permutations).
If I have a great rapport with her, some interests in common, and there is no problem with us spending quality time by ourselves, then, sure.1
Most Helpful Girl
I wouldn't go out of my way to, because that's baggage even I don't expect to bring to a relationship and it makes things harder because you can't be the centre of the relationship with your partner. Some things to do with the kid have to come first.
So long as they didn't have a crazy baby mother trying to wreck our relationship, their kid wasn't insane and the father loved and looked after them well (and wasn't just looking for a babysitter) then I would consider it if I really really liked the guy. Otherwise no1