Contented as a kid (2,4)
I believe the answer is:
at ease
'contented' is the definition.
(thesaurus)
'a kid' is the wordplay.
'kid' becomes 'tease' (both can mean to mock).
'a'+'tease'='AT EASE'
'as' is the link.
(Other definitions for at ease that I've seen before include "resting" , "Relax - that's an army order" , "Army command to relax, stand with feet apart" , "with no worries" , "Calm" .)