Weddings. They cause so much angst and so often result in people behaving atrociously. And they're boring to boot!
Normally I have no truck with bridezillas and groomzillas who treat people appallingly during the run-up to their wedding and expect everyone to dance attendance on them. However, the bride is the one who has my sympathy on this occasion.
Reading between the lines of OP's updates there's a clear dynamic going on here with the younger grandchild as the favoured one. The 'mini-wedding' idea is - to put it mildly - unusual. This is all I think it's necessary to say on that point.
The grandmother would prefer to babysit the younger grandchild than attend the wedding of the elder one, under the not uncommon principle in this day and age of having a child-free wedding. Normally I'd suggest that if people issue child-free invitations they should expect some of the parents of said children not to attend. The grandparents, who then implicitly turn this into a choice of 'invite my golden DGC or I don't attend either', not so much. I wouldn't mind having a wild stab in the dark, too, and guessing that this dynamic is part of the reason she isn't wanted there, and the easiest decision has ended up being not to invite children at all.
Your ploy is transparent, OP, and it's unkind. One of your younger granddaughters parents could easily stay with her whilst you attend the wedding with your son or daughter. Your treatment of your poor elder granddaughter is awful. (And I suspect she's experienced similar since the younger one came along).
So pleased we eloped. We'd do the same again. What is it about weddings that makes people behave like this?