We have run into the same problem, so we do the kid swap, where one person keeps the kids, and the other rides. The give you a fastpass paper with no return time on it, and when your spouse comes out, you get in the Fastpass line and ride... have to do it all of the time on Tower of Terror! DD likes RnR now that she is tall enough to ride it. It has never bothered either of us to ride alone.
You didn't mention how old your kids are, but you could do one of the kids' clubs. The Sandcastle Club at the Beach Club Resort is very nice, and I believe opens at 5 pm... and the Neverland Club at the Polynesian opens at 4 pm. Drop the kids off there (they will feed them dinner), then go to MGM and ride. The lines are usually shorter that time of day, anyway.
We also have been known to take vacations to WDW without DD... that way we can get our thrill ride, PI and fancy restaurant fix. You know, a bit of adult fun!