Debit cards in theory do not allow any debt, thus there's not so much of a problem allowing under 18s to have them.
See this UK account for 11 to 18 year olds which allows a debit card.
(http://www.rbs.co.uk/personal/current-accounts/g1/young/revo...)
HSBC give you a savings account when you're seven, and add a current account when you're 11, with a VISA debit card.
(https://www.hsbc.co.uk/1/2/current-accounts/under-18-bank-ac...)