It's mostly oxytocin, but it's both. I was just reading about this from an old issue of Mothering, so I hope memory serves.
Oxytocin is the hormone that gives you an "in love" feeling. It's released when you are holding hands with someone, when you're making love, when you're nursing. It's the reason you sort of get an "afterglow" feeling after a nursing session.
Prolactin is the "mothering" hormone and it's less well understood, but it brings on feelings of nurturing and sacrifice and submission. In animals, the least dominant members of the herd have the highest levels of prolactin.
So put the two together and you have the mother / child bond...