To be dismissed for a one-off incident, it would have to be sufficiently serious for it to be regarded as 'gross misconduct'. Serving alcohol to under-18s
might fall into this category.
Many employers might specifically tell their employees that serving alcohol to under-18s is a dismissal offence but an employee who was sacked for doing so could still challenge the decision on the grounds that it they'd made a genuine misjudgement on a single occasion. If the employer knew that the employee was normally extremely vigilant, it might be hard to prove 'gross misconduct'.
Further information about possible dismissal is here:
http://www.ukonline.direct.gov.uk/en/Employmen t/Employees/RedundancyAndLeavingYourJob/DG_100 26619
(Under 'Where to get help', you'll find the phone number for the ACAS helpline. If you want to discuss your situation with someone, that's the number to ring).
I think that you should tell your boss how upset and worried you are through what has happened. At best, you'll find your employer supportive and you can stop worrying. At worst, you'll know that you might face dismissal and you can seek the help of ACAS in challenging this.
Chris