The parents sacrificing themselves to save their child is probably the one exception to that rule. While self preservation is almost always the prevailing drive, in a situation like that, where death is almost certainly eminent, species preservation, in some people, comes first, and the parents do what they can to save the child. I'm not saying that parents are obligated to do that, it's just instinct in some people.
Personally, if I had a child, and was in that situation, I'd probably put myself first. But then again, I'm not a parent. Only when someone is a parent can they know for sure what they would do in that situation. I'm just assuming.