My sister always recommends Medised but....I must admit we never did such a long flight so early, but dd flew a number of times before she was 2.
Try to feed or dummy during take off and landing to help their ears pop. With dd I always found the "gentle hum" of flying made her sleep most of the way - obviously won't be case with 12 hour flight though. Can you fly overnight when dc would normally be asleep?
Other than that a bag full of stuff. Snacks / drinks / little toys/books etc - somehting new for every half an hour or so? (nothing too fiddly as it is a PITA to find little things when they get dropped on the floor.
Going for a walk about - turbulence permitting. Will you get a seat back TV? That might pass a bit of time.