I don't think this is a biological imperative at all, rather I think it's just that experienced parents worry less. The first child teaches them that they can mess up a bit and the kid will turn out just fine. The second kid teaches that even more. Each successive kid gets less obsessive care not for biological reasons, but because the parents have become more confident in their parenting and more aware of how indestructible little kids can be (obviously not literally, but humans are quite resilient).