There was a thread on here quite a while back where DGPs (OP's DPs) had left a substantial amount of money specifically for private education for OP's children. OP had one sibling, who was adamant that she was never going to have children. The DGPs died and then OP's DSis had a change of heart and did end up with a couple of children. She assumed that the money was for the private schooling of all of the DGCs.... except that firstly, there wasn't enough to pay four children's ways through school and secondly, OP's children had been specifically named as beneficiaries. It caused a huge amount of pain.
It's far from uncommon to have DGC who are born after you have died - in the above situation, additional DGCs could equally have been OP's younger children, born after the DGPs had gone, and that would have been heartbreakingly unfair.
We actually knew a family where the surviving DGP decided to leave everything split 9 ways:
DS1, DIL1, DGC1, DGC1 (family 1) - 4 ninths each;
DS2, DIL2, DGC2, DGC2 (family 2) - 4 ninths each;
DS3 (happy bachelor, no DC) - 1 ninth.
None of them particularly well off (but nowhere near the breadline).
Contrary to what you might imagine, DS3 (a very kind, generous man) thought this was an excellent idea and completely fair.
DS2, on the other hand, was furious as, rather than his family receiving 4/9 of the money, he wanted to have 1/3 (3/9) - less - but all to himself.
I really don't see any fair alternative to leaving equally between your children only, unless there are very specific reasons and everybody is in agreement (e.g. one child is severely disabled and will need lifelong financial support - but even then, you can't know if one of your other DC will have a massively life-limiting accident after you have died). If they so choose, there's nothing stopping any of your DC from passing it all straight on to their own DC if they want to.
Selfishly, I think there are certain benefits to only having one child - makes things like this far more straightforward!