She purchased these items for the baby right after she was born
Whether these items were gifts or not depends on what was said at the time. Have you asked your son whether she bought those things with the understanding that he would pay her back at some time in the future?
If the items were gifts, then you have a right to keep them. The law does not look kindly on the idea that gifts can be taken back later whenever the giver changes her mind.
It's possible the whole thing will blow over, since it's probably driven by her anger at your son over the breakup. Maybe you could try meeting her, just the two of you without your son there, to talk about it. That may be enough to get her to realize that you and the baby are both innocent victims of their relationship problems, and you can work something out amicably. If that matters to you, of course.