Sorry but I think this is a bad idea in itself.
Not all 'kids' want jewellery. And what they want at 12-16 won't be the same a 25/ 30/ 40.
It's a very different matter being given an heirloom (which, IME often isn't too taste and lies in a drawer for another 40 years....rinse and repeat!)
Why vouchers? Never heard of this as an insurance payment. It suggests they are for a 'chain' like Ernest Jones or similar.
Why isn't she being reimbursed with cash?
I can get my head round why the gran thinks it's a good idea in theory- heirloom was stolen, so replace with the same items. But in reality it's not a good idea.
As a compromise, maybe you could keep the vouchers till your kids are 21 and then allow them to choose something - but (not being nasty here) the amount per child isn't very much. They would get around £650 each but that's not going to buy a 'classic' watch like a Rolex, Omega, etc. Or a very big diamond. I paid £1500 for a small 0.4carat diamond pendant many years ago.
Is she open to a re think?