Might I suggest a small pail with a stuffed animal, a few good for you snacks and maybe some sidewalk chalk and a small age approprate puzzle. The only issue I can think you may have is the occasional child with food allergies, you may need to make sure that the snacks come from the health food store and do not inclued gluten, milk, nut or wheat products. The other route would be to ask for any food allergies with the invitations R.S.V.P.
Hope this helps