I don't actually know but if you're looking for a seven letter word which is
NAUGHTY...
hellions
Scamps
Imp.
Scamps.
heathens, hellions
the letter is u
A mischievous child is called unruly. It begins with the letter u.
Idle, idol, inch, inky, itch, iced, iron...
imp
pixie
Rascal.
Brat.