Contrary to popular belief, Premium Bonds are not safe unless you're winnings beat inflation, and the majority of Premium Bond holders do not beat inflation. Pattree's comment is a perfect example. In 1960, £10 probably wasn't far off a weeks wage yet today It's less than an adults minimum wage for one hour. The number of GBP he 'invested' in 1960 might not have gone down, but it's value has, and the end result is the same, less purchasing power.
Premium Bonds might make more sense as a short term savings pot when interest rates were near zero but when you can get a guaranteed 5%~ they make a lot less sense. long term, they are a very poor investment choice. They are an especially poor choice for children who have the advantage of time and with time comes the opportunity for exponential compounding effect.
Long term, my choice would be to diversify across multiple indices, and physical precious metals. Gold has actually been beating the returns of the indices since 1999. I buy Silver and Gold bullion as often as I can afford to. Gold is a better investment choice as it's VAT free. There is now VAT on Silver since Brexit. But you can buy a 1oz Silver coin for a little over £30 so they are a bit more affordable. I buy 1oz Silver coins for my niece and younger sister for Birthdays and Christmas instead of toys.