ITA! Not to mention, that hopefully the families would be saving the money for the child's education/future ect., so obviously it wouldn't be fair for 3 children to have to share the same amount of money that you gave to the family that only has 1 child ect. Personally, I think the fair thing to do is to give each grandchild a set amount of money.
OP never stands the grandchildren are minors.
When DH's grandmother died, her will stated 10% of her money was to be evenly divided between grandchildren. MIl had only 2 while DH's uncle (MIL's brother) had 5. Each got the same amount.
It comes down to what the will said. If there is no will then the survivors get the estate divided among themselves and can share with their children in the manner they choose.