Today's naughty children must be completely successful! (8)
I believe the answer is:
unbeaten
This clue is a double definition.
'today's naughty children must be' is the first definition.
(as in corporal punishment)
'completely successful' is the second definition.
(eg not beaten by any other team)
(Other definitions for unbeaten that I've seen before include "not having suffered defeat" , "Yet to lose" , "never surpassed" , "Never defeated" , "Always winning" .)