There is no question that the young ones may have a hard time getting back into the routine and going to childcare (after a long stretch at home with just parents). I've noticed that pattern with toddlers and preschoolers.
There are times when a child has had a bad experience at childcare and may be fussy because of that - because these situation exist parents could be a little worried about their children when they can't see what happens all day with their child. The fact that you've had this little guy a year, though, should calm those fears with these parents.
Sending pics to reassure them is a great move.