Honestly, I wouldn't be 'guaranteeing' anything at this point. Tell her that you hope that everything works out, and that as things are right now, it's looking good.
But I don't know that I would be promising, as that's a loooooong ways away!! I have the same situation. Mom went on mat leave and we discussed the possibilty of her bringing both her first and the "new" child. We left it open, ultimately.
If you are going to; I would charge a deposit, but as the ladies said, not a monthly "fee" if the spot is actually filled.