Is she supposed to let you know by today or by next Friday?

If it is next Friday, then send her an email saying, that although you know she is going to have a decision for you by Friday, if she could let you know any sooner, you would appreciate it as you have other families waiting for the vacancy, but you would prefer to give her first choice as you feel her little one is the best fit for your daycare.
I don't know how else to word it. If you are worried about your other parents, maybe send them an email saying you've had a lot of interest in the vacancy, and need a bit more time to come to a decision, but will let them know either way as soon as possible.

Hope everything works out for you!