Children appearing about noon in classes (5)
I believe the answer is:
kinds
'classes' is the definition.
(I've seen this in another clue)
'children appearing about noon' is the wordplay.
'children' becomes 'kids' ('kid' can be a synonym of 'child').
'appearing about' indicates putting letters inside.
'noon' becomes 'n' (abbreviation for noon).
'kids' enclosing 'n' is 'KINDS'.
'in' is the link.
(Other definitions for kinds that I've seen before include "Types" , "Sorts, varieties" , "Categories, sorts" .)