If I understand the OP correctly, Daddima is looking at splitting the money between/amongst her children to pass on to the grandchildren, as she will do in her will? So (for round figures) if there are 2 children, 7 grandchildren and £7000, each adult child would get £3500, and if one has five children and the other two, the grandchildren would get either £1750 or £700 depending on whether they are in the larger or smaller family.
If that is the case, I would consider that way of doing it to be less fair than simply giving them £1000 each. They are equal in their status as your grandchildren, and in your shoes I would make them equal when it comes to gifts. When it comes to your will, I think it's fine to spilt your assets between your own children, and what they choose to do with it when their time comes is up to them. Yes, that will mean those in the larger family will each inherit less indirectly from you (assuming there is anything left when their own parents pass it on) but anything could happen between now and then, and IMO that is their parents' responsibility, not yours.