"ideally I would invest my sons share as given the time line it would be expected to do better than bank interest"
The will should have said who the trustees will be. If it doesn't say anything then it will be you and your brother jointly, as executors.
You will be responsible for investing the money of all the grandchildren, not just your own son.
You mentioned that there are grandchildren under the age of 18.
A beneficiary cannot inherit anyway before they turn 18. So, if there were no age requirement in the will then the 14 year old would have the money held on trust for them until they turned 18.
But since the will mentions an age, that means that it becomes a 'contingent trust'. That is, they only inherit if they reach the age specified - their inheriting is contingent on them reaching that age (if they die before that age then they don't inherit).
This sort of trust has more complicated tax rules and there may be inheritance tax considerations and capital gains tax considerations if the total value left to the grandchildren is worth more than £325,000.
"ideally I would invest my sons share as given the time line it would be expected to do better than bank interest
is that ok and would I open the account in his name or some kind of trust"
You (and your brother if he is also a trustee and/or executor) would be responsible for investing the money for all of the grandchildren, not just your son.
Not all banks will open a trustee bank account. I bank with Barclays and they don't do this anymore. Metro Bank still do trustee bank accounts and I am sure that there are others as well. The account is opened in the names of the trustees.
https://www.metrobankonline.co.uk/business/sector-services/products/trustee-banking/
If you're investing in shares etc then most platforms will allow you to open standard accounts in the names of the trustees
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"we want to honour mums wishes and ensure the kids don’t just blow the money at 18!!"
When you say "wishes" do you mean that the requirement that they don't inherit until the age of 25 was in a letter of wishes or it was actually in the will itself?
If this was in a letter of wishes, then you can do anything you like.
If this stipulation was in the will then it doesn't matter what you "want" to happen, you are required to follow the terms of the will.
"mum left a sum for each of the grandchildren for when they reach 25"
It will depend entirely how the will is written. If the will uses a phrase like:
"I give £100 to each grandchild provided that he/she attains the age of 25"
Then they only inherit when they reach the age of 25. If any die before that age then they don't inherit and that money goes back into the estate to be redistributed according to the will.
In addition, if an actual amount is mentioned then that is the amount that they will inherit when they reach the age of 25. So you wouldn't need to invest the money, any increase in value above the figure mentioned in the will would go to the residuary estate, not to the grandchildren.
In contrast, if a percentage of the estate is mentioned (eg "I leave 5% each to my grandchildren") the you would want to invest the money.
This can all get really, really, complicated and I would strongly suggest that you speak to a solicitor so that you understand what you are required to do as executors of the will