Ms. Bazelon, this was a discipline opportunity that you blew.

Sure, it is right for you to make sure the ducks are in line snack-wise and bag-wise etc., but it is even more right for you to teach your son that life is full of speedbumps and frustrations. The earlier in life that he learns this, the more rewarding his own handling of reality will be for him.

The very structure of his tantrum was your way in: he was throwing a shitstorm because his snacks were missing. Remind him (and a slap to show him that his outburst there at soccer was wrong would not be inappropriate) that since he was able to describe the problem verbally that he was more than half of the way to solving the problem or coping with it. Show him that the tantrum served neither goal. You could even wash this down with a reminder that the only way for him to be SURE to have his snacks is to prep them himself and then to maintain the chain of custody. He'd either become good at it or would learn that such costs are burdensome; either way, he'd learn a lesson about life & balance.

You do him a profound disservice by letting him go on as you did. Don't shield him from every variable in the universe and don't be quick to medicalize his attitudes.