This would need to be tested either in NCAT or at the Magistrates. This would still incur a cost to the person seeking redress perhaps much more than the $100 lost in the deposit, but it also risks not winning and still having spent extra.
Costs the Hairdresser could easily argue for are the time involved in booking, time involved in cancelling, time spent in refilling the vacant slot (and already potentially lost custom). As it appears it may be a well renowned business this may be a high enough cost to easily be over $100 even if we don’t think it is.
They, the affected, could complain to the ACCC over the matter but chances are much more in favour that ACCC will not act, but at least to complain to the ACCC is free.
I think the easier argument to make is that it was a Minor and thus the contract may not be binding, particularly as NSW does not affirm a contract is binding when a Minor but instead only makes a presumptive call.
In all of this getting some competent Legal Advice would be a great starting point and there are plenty of Community Legal services that will provide free and good advice.
A list of some
NSW in particular
