The sensible approach is always to have a written permission from parents/guardians.
It might be possible to book direct but that does not guarantee that they will be allowed to travel. Airline rules can be applied at the airport, or, that once there (particularly if booking is made direct with the accommodation) they will not have a problem.
Just as with the postings we keep getting about time left on a passport. It is not a case of what we think. Rules are there for a purpose. It is only when things go wrong that people ask why they were not applied and so prevented the problem.
fwh