SAIM2430 | Interest: taxation of interest: children's accounts
From HM Revenue & Customs · Savings and Investment Manual
Accounts held by unmarried minor children
Interest arising on an account held by an unmarried person under 18 is treated as income of his or her parent if
the money in the account was a gift from the parent, and
the interest exceeds £100.
The gift constitutes a settlement and ITTOIA05/S629 (1) applies to treat the interest as income of the settlor. There is more detailed guidance, with examples, at TSEM4310.