There was something in another thread about a woman who always brings her teenage son, whether or not he is invited, and even when the invitations specify an adult event (cocktail pary) and said specifically adults only --21+. I wondered at the time why the prospective hostess didn't talk to the son, who is about 15, and explain that it was an adults-only party, there would be no other teens/young adults present, he would be bored silly, and he wasn't invited not because the hosts don't like him, but because they DO! The event would not be appropriate for his age and interests, and he wouldn't have any fun. They look forward to seeing him at other times, of course. Could he please explain that to his mother and plan something else for that evening that would be fun for HIM? Don't know the boy, but thought it might work.
Same applies here, more or less. Maybe OP could ask her kids what THEY would rather do, once the invitations come out and the options are clear. They could maybe go to service and then blow off the fancy, dress-up restaurant with no other kids and all grown-up behavior, or decline the whole thing and stay home and have a movie fest with friends or something else they might enjoy, and send Uncle and New Aunt a nice card. Or maybe they will be asked to everything, and they, or either of them, might WANT to go. Then, there are alternatives the children and OP could propose to DH that make sense for all.
Note, too, that church services are generally open to the public. Invited or not, the kids could go to the wedding service and then cut out for some more age-appropriate activities.