My two kids have some premium bonds. We were discussing what would happen if one child won a significant amount.
Imagine one child won 1 million - what would you do? I think we'd split it 3 ways - 1/3 each for the kids and 1/3 for us to give them a better childhood (travelling, activities etc).
As far as I'm aware any significant wins get paid to the parents to do with as they see fit.