That is another tough part. If not participating in youth FB doesn't allow the kid to get playing time in junior high, and thus does not allow him to get playing time in high school, that is clearly a lost opportunity. If youth tackle football was eliminated altogether you would do away with this problem. No kids would have "head starts" on others, everyone would be putting the pads on for the first time in junior high.
I do feel that more often than not a kid will not be stunted at all by not playing tackle football in elementary. If the kid knows how to run, catch, and throw, and someone takes the time to teach them how to block and tackle (without pads, without collisions), they will be caught up with their peers in a matter of a few practices. We probably grossly overestimate how much kids actually learn by participating in youth tackle football.