Wow, I SO wish my DSS' biomom was like you. Congrats.
After skimming through some replies, my only advice is to keep at it (meaning communication). And keep reiterating that the only reason you're doing this is to keep things friendly and open for the sake of the children. Finally, I'd make sure that you respect how your children's new stepmom is going to grow into her relationship with your children. At first, she may not want to talk to you, but perhaps that'll change later, and if you keep the door open, she may walk through eventually.
I have a stepson and I've been in his life for 6 years now. In the beginning, his mother just outright hated me, probably because my DH had just been involved with a very unstable woman with whom he believed he had a child. My poor stepson spent two years developing a relationship with a little boy who turned out not to be his brother (the woman slept around like no other!). The separation was horrid for my stepson and there was lots of anger left over from it.
When I came into the picture, biomom was understandably afraid that I was another promiscuous girl who'd hurt her son...she constantly asked my DH if I was pregnant or if I planned to be. At heart, she's not a very friendly person, so I found her demeanor difficult to deal with. DH and I proved to be stable and I thought that she'd realize that I wasn't a freak who'd hurt her son. Quite the opposite, I started trying to help him in school and do some fun activities with him, kind of like a big sister figure...but then biomom hated me even more, because she thought I was trying to replace her.
I WISH we had been able to have an open and frank discussion about our respective places in my stepson's life, but she was way to insecure and uptight for that. When biomom got married and pregnant with twins, I made her babies little bunting bags and wrote her a card, saying that I hoped we could be friends. I never received a thank you or any acknowledgment that I'd done anything for her. It still stings to this day.
Eventually though, DSS grew up, DH and I continued to be stable and we moved to another province, taking DSS with us. For some reason, distance made things a lot easier. Now DSS's mom is very cordial over the phone with me, and even took the time to return my call when I offered to go pick up DSS' train tickets for him. So we're not the best of friends, but we can talk. I still wish we didn't always have to go through DH for stuff, but she's never shown a willingness to talk to me so I try to respect her.
Good luck and I hope this helps!