Mastiles_fanylion – you can have up to £10,200 into a ISA in any given tax year. Any growth is tax free as are withdrawals. The risk element depends entirely on where the funds are invested, I have clients who have a huge range of investments ranging from cash (low risk) to off the scale (usually far east equities)
ISAs are not just for low risk!
As for the OPs question; as already said, any cash you are setting aside should go into a portfolio that YOU control. Keep it’s existence secret from your child, so you release the funds to them when you deam it appropriate (maybe for uni or when they need a deposit on a house etc.) The CTF matures automatically and there is no control over what it’s spent on.