From HMRC website:
If your child goes to live with someone else, you may be able to keep getting Child Benefit for up to eight weeks. You might be able to get it for longer if you keep contributing towards your child's upkeep.
If your child leaves home to live with someone like a friend or relative, the Child Benefit Office may keep paying you Child Benefit for the first eight weeks. It may be less than this if the person your child's gone to live with also makes a claim for your child.
You may keep getting Child Benefit for more than eight weeks if:
- you're contributing towards your child's maintenance and accommodation
- you're contributing at least as much as the Child Benefit you get for your child
- the person who your child's living with hasn't claimed
All of these must apply.