I've seen this on the tax credit website:
"If your circumstances change, you will need to contact the Tax Credit Office as soon as possible, as this could affect the amount of money being paid to you.
Change of circumstances include:
- If your child leaves home or you start looking after a child
- A child goes to live with someone else
- Your child leaves full time education or approved training before the 31 August following their 16th Birthday
- Your child starts training that is provided as part of their job
- Your child gets a paid job, and they work 24 hours of more a week which for which is paid
- Your child makes their own claim for benefits or tax credits
If the change in circumstances are not reported, you may receive too much Child Tax Credit and have to pay it back. You may also be charged a penalty for not notifying the Tax Credit office.
It is recommend that you call the Child Tax Credits office as soon as possible to report a change in Circumstances: 0844 496 6511."