>>> You shouldn't be serving in a pub under 18
Not strictly correct. Since the Licensing Act 2003 came into force, there's now no minimum age to serve alcohol. However every sale by a person under the age of 18 has to be individually approved by a licence holder or by a person authorised by a licence holder to grant such permission. (The situation in a pub is exactly the same as it is in a supermarket, where a checkout assistant who is under 18 has to seek approval every time they permit the purchase of alcohol).
If an employee (of whatever age) is paid 'cash in hand' then, if HMRC become aware of it,
(a) the employee can be required to pay Income Tax and National Insurance on their earnings ;
(b) the employer can be required to pay the employer's National Insurance contributions ; and
(c) the employer can be required to pay any unpaid holiday pay to the employee.
If the employee was claiming benefit (and not declaring his/her earnings from the pub) then he/she can
(a) be prosecuted for fraud ;
(b) required to pay back any benefit which they weren't entitled to.
However the employer (who was also breaking the law by paying 'cash in hand') has no right to demand any money back which was paid to the employee.