We don't have any children who would be potentially traveling with us IF we were going down during that time, so this is somewhat irrelevant, BUT...
In reading about the new program, I've always interpreted it as something they were encouraging families with young kids to do, but not PROHIBITING families without young children from experiencing. It's just their marketing tool, and the highlights of that extra hour are designed to attract young Disney kids.
If, however, your party wants to check it out, I don't see why there should be a problem. The debate over MB is something like the "moral dilemma" over whether an all-adult party should book breakfast at CRT.
ETA: Although I do think it would be funny if they DID try to enforce it with an age restriction...we'd have to rent a kid just to enter the park!!
"Excuse me, ma'am...I see you have two toddlers and a preschooler. Seems to me you have more than the adequate number of children necessary to enter the Magic Kingdom, so would you mind if, just for a little while, I borrowed your son? Just for a bit? Has he had his shots?"