The Nutella fold-over sandwich is a brilliant and wonderful snack for kids & toddlers (without nut allergies, of course). We use 100% whole wheat bread so it's healthy. It's got protein in the Nutella, so it's filling. And it's chocolatey so the kids both LOVE it.
It should, however, be a closely supervised activity with the younger kids, and probably be confined to high-chair time.