I was thinking along the lines of Urban. If it looks like this one might be around for a while and you really are pretty indifferent, you might just ask if you can talk. Tell her what you told us - that you have no need to be besties, but you'd like to be civil for the kid's sake. Tell her that you have no interest in your ex and that your interest in her, him and/or their relationship is limited only to how it effects the kids.
She likely assumes that you hate her and bad mouth her. She was an AP and probably assumes (as you often have) that the AP assumes the majority of the blame for the break-up. She doesn't get that you have no love for your ex because SHE thinks he's a catch. So let her see that she is wrong - right from the source. She may question what he's told her about you if she begins to realize that the picture he painted doesn't match the one in front of her. Then leave it be and just act civilly when you see each other. I've known more than one woman who found that dealing with the GF (even ex-AP) of an abusive/manipulative ex-spouse was easier than dealing with the ex-spouse themselves. The two women figured out schedules and left the guy out of it. Kept drama to a minimum for everyone. May not be possible with this woman, but who knows? Good luck - this one is never easy to navigate.
BS - Female
Married 27 years, one adult child DD May 2016 “When we are no longer able to change a situation, we are challenged to change ourselves.” - V Frankl