Who can be appointed trustee if someone under 18 inherits via intestacy laws?

I'm in the UK. A family member is not expected to live long and has no will. He is in intensive care and not legally competent to make one.

He has three children ages 23, 19 and 17 and is divorced from their mother. We understand they will inherit, and the youngest (G) will have a trust fund until he is 18, which is May 2020. But his mother is in a lot of debt and the older two children are worried that she'll spend G's inheritance before he gets it. Previous experience shows this is likely.

Is it possible that the older children could be made trustees or does it have to be his mum? Or as well, with the trust requiring more than one signature to withdraw money?