Most children born between 01 September 2002 and 02 January 2011 had a Child Trust Fund set up for them either by their parents/guardians or by HM Revenue & Customs. The average value of each was £1,500 in 2020. These mature when the young person reaches the age of 18.
If you do not know where the account is held you can use HMRC's online tool to track down your provider.