What you've described sounds like an unenforceable liquidated damages provision.
In contract law, a breach that results in actual damage to the non-breaching party can be remedied by the payment of money. In fact, the existence of damages resulting from the breach is a prerequisite to a lawsuit for breach of contract.
For example, let's say that Sue hires Bob to paint her house in exchange for payment of $2,000. Bob does about 2/3 of the work and then walks off the job. At the time Bob abandoned the job, Sue had paid him $1,000. Sue seeks another painter to finish the job but discovers that she cannot find anyone to do the remaining work for less than $1,500. In addition, she learns that some of Bob's work was defective and will cost an additional $500 to correct. Accordingly, Sue will have to pay a total of $3,000 to get her house painted. Because she had a contract for Bob to do the work for $2,000, her damages are $1,000.
Liquidated damages are damages that are fixed at the time of the making of a contract. The intent is to cover situations in which ascertaining actual damages resulting from a breach would be difficult, impossible or impractical. I don't know the specifics of Wisconsin law on LDs, but it is generally the case that either actual damages must actually be difficult or impossible to determine, or it is reasonably believed at the time of contracting that actual damages will be difficult or impossible to calculate.
In the situation you described, there is no reason to believe that actual damages would result at all, much less that they'd be difficult or impossible to determine.
If you are one of the parents and you want Hannah to pay the $500 and she won't do it, you'll need to sue her in small claims court. Small claims court is (or is supposed to be) simple and inexpensive, but I'd be very doubtful about your likelihood of success.