If you do decide to keep the deposit and likely go through the small claims court route (on their end) it might be best to sit down now and document what efforts you put in during the 3 weeks to help the child adapt and also list what requests you made of the family to help the child adapt. Then if you do end up in court you can present your case this is what I did, this is what was needed on the home end but they did not attempt to help the child adjust. Therefore, after 3 weeks of effort you were left with a space unfilled and no income from the space because the family did not do their part.

Anyways, documenting it now will help later.