If kids are snacking all the time, how do they avoid getting diabetes/metabolic syndrome?

Good genes, activity. Kids who are physically active (and most kids are) expend a tremendous amount of energy. Snacking is not a problem for these kids, especially if they are free of genes which would predispose them to diabetes. It's when frequent snacking, bad genes, and lack of physical activity coincide (and that's happening more and more with video games), the trouble starts.