Brain Test Level 83 Answer
Tapping the children or dragging the cookie to one of them, which only picks a winner. Giving the cookie to either child ends the argument in that child's favour and the level does not accept it, because the fight carries on in the other direction.
The puzzle
Stop the fight.
The answer
Grab the cookie with two fingers and pull it apart into two halves, one for each of them.
What most people try first
Tapping the children or dragging the cookie to one of them, which only picks a winner.
Why that is the answer
Giving the cookie to either child ends the argument in that child's favour and the level does not accept it, because the fight carries on in the other direction. The instruction says stop the fight, not win it. Pinch the cookie with two fingers and pull it into halves, and with a piece each there is nothing left to argue about.
If your level 83 looks different
Unico Studio shuffles level numbers between game versions and platforms, so the puzzle on your screen may carry a different number from the one here. If the question above is not the one you are looking at, the full levels 51 to 100 walkthrough lists every puzzle in this stretch by its question, which is easier to match than a number.