I don't get the snack thing either. The only time I give my children a "snack" is when meal times are going to be significantly later than usual (for example, they are doing a sporting activity at about 5, so tea is much later).
When we go out with friends for days out, they produce snacks mid morning and mid afternoon (usually bags of mini cookies or gingerbread men etc). I find it odd. Their children, incidentally, aren't so good at eating their healthy lunch or tea ...
Forget the healthy/unhealthy/ less than 100 calories snack etc. Change for life should say that snacks are not necessary for anyone. I know babies are fed on demand (which is good!) but it is fairly usual for babies to go 3-4 hours between feeds, so I'm not sure how older children suddenly can't manage this anymore...