No qualified, licensed day care facility should have a problem filling vacancies in this market. Take it to small claims court. The judge will probably ask her what she did to mitigate her damages, i.e. did she fill the slot and if not, did she try to. If she filled the slot and lost no revenue because of your cancellation, she should give the cash back. Deposits are not meant to unduly enrich one party. Better yet, take it to judge Judy so we can all watch
Actually, keep a log of all phone calls and send a registered letter requesting a refund. Be sure to state the dates of all attempts at communication in the letter. It is important to document when notice of cancellation was rendered. Then there will no defense that she was not contacted/notified.
P.S. You can also file a complaint with the state against child care providers. Look for the info at
http://classifieds.somd.com/child_care_rules.html. I doubt they can make her give the $ back, but it turns up the heat.