While cash is the most immediate form of payment and accepted everywhere (in fact preferred in a number of cases), credit cards and traveller's checks can be an alternative. But there are pitfalls.
Passports issued to children under age 16 are valid for 5 years. Children under 16 must renew their passports in person and follow the same guidelines as when the passport was first issued, including being accompanied by parents or guardians. Two current passport-sized photos must be included with the application.
The cost of renewing a child's passport is Â£49 for the standard service, Â£96.50 for the one-week service and Â£109.50 for the premium service. There is no way around these costs if you want to go abroad unless you were born on or before 2 September 1929, in which case your passport is free.