There’s £50K, there was no will, so OP and her two siblings are going to inherit £16,666 each.
They’ve agreed amongst themselves to split it between the five grandchildren, that’s £10K each. They’re doing it because they “know” that their DDad wanted the money to go to his grandchildren. However, he didn’t make that legal by writing a will.
So legally, OP, you can do what you want with your £16,666. You can do what you had agreed and hand over £6,666 to your siblings, and then it’s up to you whether you keep the £10K for your existing DD, or split it when number 2 comes along.
You could speak to your siblings and say, look, can we wait and see if I do have another child, and then split it six ways?
Or you can say to your siblings, sorry, circumstances have changed, I’m going to keep my £16,666 and split it between my 2 dc if I am fortunate enough to have a 2nd, or keep it all for dd if she ends up being an only one.
And legally there is nothing they could do about it. But it might sour your relationship with your siblings....
I think it’s a bad arrangement, and a good argument in favour of making a will.