by law, you cannot ask her to pay that much money even she breaks or ruins a valuable thing. The most she can pay is up to ? dollars. I forget how much.......
i think it is not a matter of money but children safety. For compensation, you can ask her to pay for the expense, ie $300 max for each item and a total of not more than 1/4 of $3,270 per month ie about $800 per month. As bun bun is very minded about money, ask them to pay is one of the good methods so that they will be more careful next time.
Also, please consider writing them a warning letter on this incident as any negligent to children caring and depends on degree is criminal and report it to police. Of course, you may issue the warning letter to her on the pay day or after their day off , you know what I mean.