A.D.
don't stress out! Where I used to work, just about everyone had kids and I didn't. They talked about birthday parties and whatnot all the time. Not only was I not invited, but honestly, I had no desire to go hang out with a bunch of kids!
Tomorrow when you go into work...I'd basically say to her, "You know, I was thinking about it when I got home...and it's a bit awkward. I mentioned my son's birthday party to you. I assumed you wouldn't be interestd in coming to a kid's party, but I could be wrong. Anyway, just wanted to mention it, in case you WOULD like to come, you are more then welcome and we'd love to have you there."