Children ought to swallow as much as required (6)
I believe the answer is:
enough
'as much as required' is the definition.
(I've seen this before)
'children ought to swallow' is the wordplay.
'to swallow' indicates a hidden word (I've seen 'swallowing' mean this).
'ENOUGH' can be found hidden inside 'children ought'.
(Other definitions for enough that I've seen before include "An adequate quantity" , "As much as required" , "Satisfactory quantity" , "To the required extent" , "Not wanting" .)