'Tis very difficult. The only cure for Fujijunior's "let's undo the seat belt" game is to distract with something else (game, book, whatever turns him/her on).
Or get the hostie to give you one of those extension belts that attaches sprog to you - and tiewrap that! You don't mind sprog being firmly attached to you if it comes to a hasty exit.
Go well-supplied with Junior's favourite sweeties (or whatever). Otherwise, two hours of tantrums will make you extremely popular with the other passengers.
I speak as a Dad and Grandad with much experience of under-fives in aircraft, spread over a long time. The little "thread the wooden peg with the string on it through all the holes" (Copyright Lufthansa 1970-ish) seems to be a wonderful device.