Premium bonds are simple, tax-free and cost-free, but I believe your children would have direct access to the money from age 16, which is quite young.
However, for that sort of amount, you probably want to prioritise simplicity and avoid any sort of fixed costs, which probably rules out setting up a formal trust.
if the point about age 21 is not cast in stone you might consider a junior ISA as the tax wrapper. I believe this would ensure any earnings are tax free, like with Premium Bonds, and that it would also minimise admin costs, but I believe it would require the money to go to them at age 18.
Alternatively, you may be able to put the money in a separate account in your own name, and leave it to them in your will, as a way to avoid any extra admin costs. However, investment earnings would be taxable (on your tax bill) and if you were to die prematurely, IHT might apply, depending what else you own.
The above comments are all about the structure / tax wrapper. Then comes the question about what to invest in. Given that they are 10 and 13, I agree with a PP who suggested you consider stocks and shares, given that the time horizon is long enough, but only if you feel knowledgeable enough to do that. Otherwise, some sort of fixed term deposit, or premium bonds, or perhaps a pair of gilts maturing in the year of each of their 21st birthdays, might make the most sense.
This is not financial advice since it does not take full account of your circumstances - it’s just a non-comprehensive list of ideas for your further consideration.