Agreed, children learn fast and many times their first words are "no" when the parent tells them to do or to stop doing something wrong. Sin lives in us, the flesh.
My only concern here is that even though a child knows, naturally how to sin, do they understand what repenting and being born again means? I have many Catholic friends (I live in the most Catholic part of my city) and, I do not get into conversations about baptism with them. I find no place in the NT, infant baptism. What happens to a child that dies at a very young age and had not been baptized yet? It's pretty hard to believe they will not be in the Kingdom. Unless, since God knows the beginning to end and knows that child will not accept Him He will send them to Hell...still very hard to take.