Please don't assume she's had no experience with children, I'm 20 and pregnant.
i have 3 dsds, one of which lives with us and who I am a mother figure for as I do all the mum things for her and dp is very stereotypically male/ dad like.
Before I became part of their lives, I didn't have any neices and nephews because my brother is 14... but I had volunteered at men cap with the children's services between the ages of 15 and 18, one evening a week and on daytrips during the holidays, working with kids with learning disabilities. I did work experience in nurseries twice in school. I volunteered at a sure start mum and baby group for a year. This was all partly to get work experience to be a nurse/midwife and am now doing a nursing degree :) I also had/have very young cousins and friends with babies and young children who I have spent time with.
What I mean is don't assume she's got no experience with children, she might have lots that you don't know about. (and I had a questionable sex life until recently, so the two aren't necessarily linked)
Aside from that she will be young and have lots of energy to play with your dd.
I understand you will have the feelings you do against her, I've been left for someone else (albeit I was only 17 but had been with him for 2 1/2 years so it felt awful at the time) and there's no way I'd of wanted her near my child. But if they do become long term then it will have to happen at some point. Sometimes as much as you don't want it to be true you can leave a wrong relationship for the right one, I left my ex for my current dp and now we live together and I'm pregnant and intending to get married. Unfortunately you have to bite your tongue around your child and go sob in a pillow or cry to a friend! its horrible but so is life.
Just remember he's the one who did this to you not her, unless you were friends, she had no reason to be loyal to you, he was the arse hole who should have split up with you first, who knows she may have even tried to get him to! I'd rather my kids were around her than around him!