there is nothing wrong with dating her but don't lead her child on. its best to build a strong relationship with the mother before becoming mutually attached with the child. children are very fragile and get emotionally attached easily
Not the greatest idea for either of the "kids", and you are one of them. Statistics aren't on your side, so be realistic in your expectations. Parenting someone else's child when you're still wet behind the ears, good luck with that. Don't let lust cloud your better judgment. Think carefully and listen to your inner voice. Good luck.