Adults only wedding w/immediate family children

Sounds like you're trying to give the kids jobs to justify them being at the reception. That's fine to include them but it's also acceptable to invite children in circles without them being in the wedding.

I agree. You can probably do siblings fairly easily, if his are kids, and yours are adults anyway. But if you invite your nieces and nephews, I think you need to include his as well.

There is no breach of etiquette in limiting invitations to siblings and nieces and nephews. Just be sure to invite all siblings and all nieces and nephews. There is no need to invent a job for all of them.

Also, if someone asks about their children, don't use the excuse that you are having an adult only wedding, because you are not. Tell them that you are unable to extend an invitation to their children.

Thats what I was wondering about.
My little sister (out of 5 of us) just turned 8, while I am 33.
Then we have our 10 and 11 yr old kids, mr 9 yr old nephew, 5 yr old twin nieces and then an 8 and 5 yr old niece and nephew of his side.
We thought about adding a cut off age of 8, but this leaves a lot more kids than we realized invited. So then realized immediate family is a daily thing for us .... and truly important.
So how do you WORD it?
I'm familiar with the names of those invited on the envelopes, and having family spread word that its restricted to immediately related children .... but I mean, our current guest list involves people that would total children to 26+ and its just too many to allow our budget to comfortably accommodate them separately. We have thought about including an info card with the invite suite to explain this (as we dont want to do a website), along with a few other details .... but I dont want to seem rude in the least, either.