All of them are grammatically correct. However, if you're talking about a human child, you'd only ever use #1. #2-4 aren't necessarily wrong when talking about humans, but it's a bit odd to refer to a human child as "it." You may hear #2-4 in nature documentaries where they are talking about non-human children, though #3 is likely less common as it doesn't specify whether the parent is the father or the mother.

All of them are grammatically correct. However, if you're talking about a human child, you'd only ever use #1 (assuming the child is a boy). #2-4 aren't necessarily wrong when talking about humans, but it's a bit odd to refer to a human child as "it." You may hear #2-4 in nature documentaries where they are talking about non-human children, though #3 is likely less common as it doesn't specify whether the parent is the father or the mother.