Just one more perspective... The current "group date" is not at all what you proposed. (And you might not want to set a precedent of "bait and switch" working.) If you want to make sure your kids get to see grandma, that will happen regardless of how many adults go along with them.
Seems to me that MIL would be perfectly happy if just the boys showed up. Since H jumped on board, that's a perfect opportunity to say, "Great, you guys will have a fun time."
From a logistical perspective, it only takes one of you to drive the kids there. Unless you really want to see MIL and are willing to do the group date to accomplish that, then it's fine to just drop back to the main goal of getting kids to grandma's.
