She's getting a hard time because she wrote an OP about a very specific situation, then many pages in, said that actually, that wasn't the situation at all. Then got irritable with people who pointed out that this made a moral, material and practical difference.
The fact that she and her siblings, not the GCs, are the beneficiaries, has a lot of implications, moral and practical as well as legal. They can each do whatever they like with their 1/3 and, if they are choosing to pool it together to pass it all on to the GCs, they can do that whenever they like; in terms of distributing the funds (e.g. they could all wait five or more years to allow for any further GCs to appear) and in terms of at what age they make it available to the GCs (this could be at 21 or 25, rather than 18 for example).
That is a very different situation from the one OP initially described; in which those GCs living at the time of the grandfather's death were the beneficiaries of his estate. In that case, the money would legally be theirs (no options for parents to pool or split it) and would legally become theirs at 18.
My concern is that OP's children (if she goes on to have another) are being doubly disadvantaged. They didn't get to know their grandfather and they will inherit less from him than will their cousins.
That seems to have come about because OP (and perhaps her siblings) have chosen to regard the inheritance as if it legally belonged to their DCs (hence OP's misleading post here), boxing in their thinking on the matter, so failing to recognise that that idea is simply a choice they have made and which could be made differently.
For example, if each sibling took their third and OP put hers in a savings account, then, if she has another child, each GC has an equal amount. If she doesn't, she can share out her 'extra' £6,666 among the other four GCs.