Cash
Cash CTFs and JISAs do not charge fees as banks make money by paying savers less interest on their cash than they charge to people who borrow from the bank.
Cash CTFs/JISAs give certainty as you know that the savings will grow by the interest rate but inflation can reduce the effective value of your child's savings.
If the interest rate on your child’s savings is below the rate of inflation, the value of the savings is potentially dropping. This is because you will be able to buy less with the money in future than you can now.