@LakieLady
My nightmare scenario is DC going off the rails at 18 having been gifted such a huge sum while DH and I are still working to pay off the mortgage we took out to ensure DC attended the best school possible.
You can (and should) review and change your will regularly - some say every 5 years, some say every 10 years, or whenever there's a relevant change.
So your will when your grandchildren are 5 could (and should) be very different to when they're 25! Most people would set up some kind of trust to protect the inheritance for minor children, and I believe some trusts can last to age 25, and some more specialist trusts can last longer.
Agree not particularly wise to leave your directly to your currently aged 5 grandchildren can access without restriction when they're 16 or 18 as you (nor their parents) will know their personality at such a young age nor how they'd treat it a decade later. But when they're teenagers, or 18, or 25, or 30, the grandparent should have a pretty good idea as to whether they'd waste it or not, so their will would be very different and if trustworthy would probably be an unrestricted bequest and a trust would probably be unnecessary.
Also, of course, as the years pass, your own adult children will be going through different life stages, and whereas you may have wanted them to inherit when they were working and struggling in middle age, their needs may be completed different a decade or two when they're approaching (or in) retirement and no longer "need" the money. Another reason why wills need to be reviewed (and changed) regularly at different life stages.