are they dead or do you plan to kill them ?
even instantaneous death ( plane crash) has a statutory solution - eldest deemed first- kinda sensible.
then the intestacy rules are applied, - dont the children get the second 250k in trust? and then the intestacy rules apply for the second death
work that out and see if there are IHT issues - there may be
basically - the children inherit in equal parts.
and the non residents can boot out the sitting child.
It is much much better to get them to write a will.