Antonin theres a lot of assumptions in that post
The assumption the children will be happy to wait 20 years.
The assumption there will be equity. The assumption the parents will be able to remortgage or sell and downsize.
Then the kids want their money. Do they force their parents to sell their home? Force them into rented.
And what if there is no home?
a) if it's legal it's not stealing, and b) depending on what the will says the OP may very legitimately be able to use the money to buy a property.
Its theft if you intend to permanently, deprive someone of what is theirs.
So given the OP can not guarantee that she can repay it, and its likely she cant. The kids could claim, when they are adults, that they should have reasonably known they were depriving them. Or that since they knew they woildnt be able to remortgage, they are intentionally deprived them.
A spare £300 per month isnt that much.
Given the op will have to save half of that to give any chance of paying it back and theres the up keep of the house, there isnt going to all this spare money she thinks there is.
It comes down to the fact that this money is the kids money and need to be available for them at a certain age. The parents may be able to do this legally. But if they cant not save the money (due to reason they cant predict) this has potential to ruin family relationships or end up in court.
And on top of all that, op has seriously thought about not telling the kids at all. That's definitely intention to permentaly deprive them.