For what it is worth, I contribute to a Junior Stocks & Shares ISAs for my grandchildren. These are opened in a child's name but operated by parent or guardian. Any money given belongs to the child and cannot be repaid. There are risks in stocks and shares but, for a long term investment, I think this is the best option. I have favoured investment trusts that invest in smaller companies at home and abroad. If the funds gets big enough, I will spread the risk and try to diversify. This is not a recommendation, just me sharing what seems sensible to me.
Sadly, index-linked savings certificates are no longer available from NS&I. When they were, they were the best bet around, providing a real return guaranteed by HM Treasury. Adults can still roll over existing Certificates but children had to cash out or switch to another NS&I product. Great shame!