If a parent bought some premium bonds for their child, then subsequently needed the money again, they can’t just cash them in and get it back can they?

I think they can

And use it as they see fit?

yes they can.. under 18’s have to have an adults name on thier holdiong and that adult can do as they wish with any bonds or prizes.. we have tidy sums in my wife and I’s names and as kids has come along they have had premium bond bought regularly.. one even won 6 £50 prizes in one month ( i wish i was as lucky)

Sorry to labour the point, but would rather get this right. If the parent has some money (which is more than their own premium bond limit) that they wish to use to pay off part of their mortgage, but due to the terms of their mortgage they are better off waiting 18 months, they can put it into premium bonds for their kids in the meantime, then cash them in at the right time to do that? Legitimately.