Invest for great grandchildren

You can invest it in your own name and not the children's name, then you have full control when they get it e.g. at 21, 25, 30 etc.

If you are worried about passing away before that point, make the provision in your will to be for your children not your grandchildren. As they have all told you that they regretted having access to it at 18, you can be confident that they will not give it to their children (the grandchildren) until the grandchildren are at an age where the parents perceive that it will benefit them most.

Or you could carry on with the current will where the grandchildren get it when you pop your clogs, and trust that their parents, who all agree that they wish they had not frittered away the money at 18, will educate their children in money matters or at least have instilled in them the commonly decent idea of respecting their parents and grandparent's wishes in relation to the gift.

If you are unwilling to have faith in the children or the grandchildren, you could use a solicitor to set up a legal trust document - for example to only release funds to the beneficiary of the trust on the say-so of the trustee. The trustee(s) could be the parents or some independent person that you pay each year out of the trust assets for the service. It is a complicated way to do it and the trustees are legally bound to act in the best interests of the beneficiaries which can be something that causes a headache in terms of the most appropriate way for the money to be invested from one year to the next. Unless the amounts are massive and big tax bills are resting on it, it's certainly more straightforward to have the parents handle the money as if it were their own, or use a simple bare trust arrangement where the money beneficially belongs to the children from the point of your demise and can be controlled by them at age 18.

Something seen from time to time here is that people often try to impose complicated rules on inheritances (which potentially leave unforseen headaches for the survivors) because the donor has some antiquated notion about people not understanding how to behave until some arbitrary date, or age like 21 or 25 etc, because in the old days you weren't an adult until you'd been able to join the army and drink and smoke and get married for half a decade or so.

My view would be that:

Your children had a life-lesson from spending their money relatively early, so you could give them the choice of either:

1)Letting the kids have the money at 18 but educating them about that life lesson, in the hope that the money is not blown, and if it is, that's fine, because the kids will also learn the life lesson first hand. Or;

2) Letting the parents assume the kids would make the wrong judgments at 18 and so keep the money under the name of and complete control of the parents until some later date that the parents are happy with at the time (let the parents set the date)

If your conclusion is that 1 is better than 2, that's fine. You could put the money in junior isa products or other arrangements that give access at 18.

If your conclusion is that 2 is better than 1, you could give the money to the parents in your will, or now, and let them handle it.

If you don't like either 1 or 2 then you will need a solicitor to create some legal wording or trust deed that handles the arrangements for the kids getting the money "late" despite being adults; as there are no obvious retail savings or investment products that have "person reaching age 25" as the maturity date.