Howdy --
I've flown 6-7 trips to Japan with my wife and child (age 0-4) with mixed-class seats. Good luck switching between cabins -- I haven't been successful yet. They'll usually use the "FAA regulations" excuse to keep you put. I've always been polite when I've asked, and the FAs get pretty snotty when I try to sneak my child back/front like you described.
Better off buying 3 seats together.
Good luck.
Austinstick