No, I wouldn't refund anything if they pulled out early in a notice period but then again, my contracts are very, very clear that fees are due during a notice period regardless of attendance.

Read your contract carefully. Is there anywhere which suggests that non-attendance does not incur fees? i.e. if you don't usually charge if a child is off sick or on vacation, then it's not unreasonable that the parents are expecting non-attendance during the notice period to be free and therefore any fees already paid should be returned.