If you are close, it is totally appropriate to give her something small (don't listen to the Debbie Downers). I do agree with PP that it should be something child related though. I usually give picture frames with some cute shots of the kids in them or a framed piece of LO's artwork, depending on MB just baking some cookies/cupcakes could be a really great gesture as well. I usually present it with a card from myself though, so it is sort of a joint gift from LO and me. I think its great you have an actual relationship with your MB and feel comfortable acknowledging important moments in each other's lives-hopefully she is as great when your bday comes around!
I agree with this. Of course you aren't obligated to get your employer a gift, but if you are close and want to do something, I think that's great.