Best cabin location on the Magic?
Correct. The best cabin location is "on the Magic."
We've had cabins Forward, Aft, and Midship, on decks 6, 7, and 8. We've not yet been disappointed by these locations. Everything was easily walkable, even using the stairs with our niece and nephew (9 and 7).
With kids, I'd recommend Midship or Aft... as high a deck as you can afford. Mickey pool, drink station, quick service dining, and buffet are 9 aft.... Movie theatre is 5 aft. MDRs are 3 aft, 4 aft, and 3 midship. Family pool, tunnel vision, arcade, and the new AquaDunk is 9 midship. Kids clubs are 5 midship.
The stuff at the forward end of the ship is more for adults (with the notable exceptions being the WD Theatre and Sports Deck).
I haven't noticed a huge difference in ship movement in the locations we've stayed. If you can, stay a couple cabins away from the elevator lobbies and laundry rooms (due to noise). Check potential cabin locations using the "stateroom reviews" option on DIS.