What does surprise me is the number of instances upthread of posters referring to other women as 'bitches' and 'cows', whether that happened to refer to the OP or her grandchild, and the OP once (unprompted) she wasn't living up to one of these unattractive pejoratives. Delightful.
I'd be curious to know what it is about weddings that has such a tendency to bring out this behaviour in people. Maybe someone somewhere has done a study into it.
The decision is no children. We can't know - the OP hasn't told us - what precisely has prompted this decision or how it would affect the bride and groom if they start making certain exceptions. It could affect their budget: more importantly to them it could cause tension and division if they're seen to be making unfair exceptions for certain other children and not for them.
I'm not a fan of rules like 'no kids' or 'no ring no bring' (the only four guests at our overseas wedding were a couple and their two young kids). But people generally take these decisions for a reason, and with careful thought. A 'no children' wedding will result in some parents not attending. This is understandable and unavoidable - where issues arise here is when the B&G take gratuitous offence when people do decline.
There's no indication anywhere on this thread that the granddaughter in question is the only child on the bride's side of the family.
Perhaps the bride is also fed up of playing second fiddle to the favoured grandchild. The grandmother in this case has made it more than clear where her priorities lie.
I have no patience whatsoever with bridezillas. There's a lot we don't know about this story, but from the little that's been shared here the bride is the one who has my sympathies here.
Incidentally, the Golden Child dynamic doesn't do many favours for anyone either at the centre or periphery of that dynamic, and that includes the Golden Child. If OP wants to create a family rift out of a perfectly usual and reasonable decision over the hosting of this wedding, that is 100% her choice.