Last night at din­ner, my first grader described the big cup­cake that he ate yes­ter­day at school in honor of a classmate’s birth­day. Between bites of pasta mixed with spinach-walnut pesto, he offered a detailed account of choco­late cake, colored sprin­kles and a copi­ous pile of frost­ing. I, of course, couldn’t help but wince. Because, you see, his class had chowed down on cup­cakes just last Fri­day. And over the week­end, he’d attended a birth­day party with (you guessed it!) more cupcakes. Yes­ter­day morn­ing, in another part of the school, the monthly Food and Fun class for the...