I'll be honest, I still have angry feelings over it all and when directly confronted it is difficult for me to bite my tongue.
Try to see her point of view.
Her children are her family--her children are -your- family. Their connection to you & your DH is the same connection as hers. Oh, sure, it's one level away, because they're the next generation, but to most of us w/kids, we consider our kids to be valid members of the family no matter what their age.
So when kids are omitted from an event, it's not that odd to see it as a rejection of their family status. To many people, they see weddings as being partly a family event. "We see them at weddings a funerals," right? Which lends credence to the idea that these are family gatherings, in a way.
Also, there's a subtext to wedding invitations: "We've invited the people who are important to us." But we didn't invite your kids--it's sort of logical to take that to the next step, which is, your kids are not important to us.
What about the idea that this is an event for adults, not children? Well, that's an abritrary decision by the wedding's hosts, not some generally accepted societal norm. Many, many people have big, fancy, expensive weddings that run late into the night and involve formalwear and -still- invite children. So excluding kids is a deliberate choice; why wouldn't someone judge you for that specific decision?
In a way your offer to include them later sort of underlines the idea that it was her children you were rejecting, because if you truly wanted an adult event, you wouldn't have offered. If you include any other kids, then of course she'd be upset. After all, her kids are your family, and if you are going to know her, you'll know them. It -is- different from a coworker's children.
Now, I'm not saying kids have to be invited, etc. You did nothing wrong.
But you might find yourself less pissed off if you can see the other side.
AND, it might offer you a tactic for what to say if she -does- bring it up: "We're sorry to have offended you; it certainly wasn't any slap at your children, nor was it a rejection of them. We just chose to have an adult event. It was very difficult to deal with the space limitations; I had hoped you would understand."
And some other phrases: "It would be a shame to let this destroy our entire relationship
. After all, we're family." (throw that back at them!)
Or just, "yes, it was most unfortunate." And leave immediately--don't you have to go potty? Don't let her suck you into discussing it.
Always be aware of your audience. You don't really give a fig what she thinks. You just want all the OTHER far-away family members to remember that you were nice and pleasant, and Aunt is crabby and complaining. Who are you going to believe in that situation?
Also, is it possible other family members didn't come to the wedding mostly because of the distance, and not because she "got them on her side"? They may not have been offended at the exclusion of the kids. But they also may be opting for low drama and so just didn't bother to explain their point of view.