Lots of good advice, but I would also put in that I generally prefer "Adults only" or variations thereof as opposed to "no kids" One is defining an inclusive group and is a bit more positive, the other is defining an excluded group and therefore slightly negative.
I also agree with some of the PPs who are suggesting more of a phone invitation, and starting out with "can you get a sitter for xyz date?" or offering to share the sitter (if you're willing) is genius!
Anyway, if you're going forward with the email I think the etiquette-ly thing to do is not say either variation in the invitation. The message of no children is by specifically naming the invited parties and omitting the uninvited parties. I'd mention the following:
- Send a specific email to the couple (so it doesn't need to be general) - and you don't have to make it obvious that this is a forwarded/forgotten invite!!
- "Mr-never-leaves-kids-@-home, and Mrs-never-leaves-kids-@-home, we would be so glad for you join us on...: "
- "We're looking forward to seeing the two of you, and would like to know by XYZ date so we can make reservations"
If, when they call to let you know, you can just "clarify" that "both of you are coming? Great, I'll add two to the guest count!" If they try to correct you, THEN is the time to break out the whole chatty bit about sitters and leaving K at home because you hoped to have an adults-only event etc.