Inheritence

If it is invested and tied up in a bond until they are a certain age (`18? 21?) it will get a decent level of interest, hopefully more than inflation which is currently at 3% a year. By the time they can cash it, they will be "all growed up" and able to decide for themselves how to spend it.

Hilda, my father died last year and left his small estate to be divided amongst his 3 children. My dh and I gave an equal amount to our two dc and a smaller amount to the four great grandchildren. My brother and sister did the same for their families. I never knew any of my grandparents and felt this was fair all round. Of course my dc children were delighted with our decision.

Why can't you just invest their share until they are old enough to take charge of it.

Oh dear, if your MIL didn't specify what she wanted in her Will then it's up to your daughter what she does with her money. You could try and prick her conscience!

You can’t , except in how you apportion the share you are passing on.
The terms of your MIL’s will are being met and if she left it to your discretion to give a share to their children, that is how it must be. So you decide how you divvy it up, you don’t say how old the DGC are but you could set up a Junior ISA for each with part of what you are allocating to your DDs. That would be fair and the DD who WOULD pass it on is unlikely to object.