As previous posters have suggested, you should invite her and her child over to your house, or offer to babysit so she can go and do something, or offer to have your husband to watch both kids while the two of you go out and do something. I would offer those suggestions a few times and if she keeps saying "no," then let her know that you're available if she ever needs help and then let it go and don't worry about it.
Also, and I say this very gently, you've had some trouble in understanding work relationships before, so remember that just because she's your co-worker, it doesn't necessarily mean she's your friend, so continued refusals may be a reflection of that.