Your best bet is to call them. I will list the number that I found
or try the links below:
Child Benefits. Phone: I have this number for them 08453003900
http://www.direct.gov.uk/Parents/YourMoney/YourMoneyArticles/fs/en?CONTENT_ID=10016699&chk=8zDeix
Child Tax Credit - Try the above number, if it is wrong then ask them and they will guide you to the right department.
http://www.direct.gov.uk/MoneyTaxAndBenefits/BenefitsTaxCreditsAndOtherSupport/fs/en