These should be required for those kids that love to kick the back of the seat in front of them.
Better yet—remember the old days, where they had smoking and non-smoking sections of aircraft? How about a kid and a non-kid area? Chances are, the business people who need to get something done while on a flight could justify an extra 10% surcharge for a seat in the preferred section. And not having to listen to whining is one additional benefit that the harness you picture wouldn’t do anything about.
This serves the purpose of handling little brats.