I personally think that the kids should be fully clothed when out in public.
A diaper is the same as underwear except it is used to catch poo and urine and no one wants to see a kids diaper.
I know there are circumstances where the baby might have had a blowout but that doesn't really happen with older toddlers.
If you don't go out in underwear then cover the kids diaper up with pants or shorts or something.