Many of the snacks I've seen young kids eating, both in school and out, are lacking in anything but carbs (in the form of sugar), trans fats and calories. When ds1 was in kindergarten, I ran into another mom in the grocery store. I was in the candy aisle (indulging in one of my binges, as I do have food issues), and she said, "yeah - just picking out a snack for M's lunch box". She was buying full size chocolate bars - around 250-300 calories each, and almost all of that sugar. I mentioned it to ds1 (not naming the kid) and he said, "yeah - about half the class brings that kind of stuff". DS1's snack was usually a half cup of kiwi chunks, or broccoli florets. One of his friends brought an apple, orange or banana every day. The kind of snack makes a big difference.
And, fwiw, the skinniest of my kids is the one who has always grazed, barely eats full meals, but snacks multiple times a day.
My ds is skinny (as opposed to dd who I'd just call slim) and he is a grazer. I also see what a lot of children at their school bring for lunch and my children bring it up from time to time.
I see that the "big breakfast" idea keeps coming up as if it is "the" solution. My 6yo dd has never been an early breakfast eater, it gives her a tummy ache; actually we are all like that, but dd is the "worst." It is a challenge to have her eat enough that her stomach isn't making noises before lunch. In the past I've packed an extra granola bar because she was hungry once she got to school, though she's also not eaten at snack time because she wasn't hungry .
I finally lost 15lbs that I'd been wanting to loose for a couple years and the key to keeping it off (in addition to cutting portion sizes, a lot less sugar, and little to no bread) has been healthy snacking so I'm not "starving" at meal times.