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