I know from experience to specifically say, " Hi 'Jane', I would like to invite you and 'Dick' over for a dinner party on Sat. night. Will that work for you?" You could include "Jack and Jill will be coming to the dinner party,as well."
You MUST specifically say their names or sure as sh-- they'll think the young 'uns are included. I know this will happen! Also if you say 'dinner party' it sounds more "adult"-like.
If that isn't enough of a clue (gawd only knows, some people still don't get it! DUH), one or both might say something about the kids coming as well - you could say "it'll be so nice for the six of us adults to have a wonderful evening together."
Trust me, you have spell it out so damn clear to some people.
Please keep us posted. It will be so interesting to hear how things go

.
Best of luck.