heathens, hellions
Imp.
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?
mischievous
the letter is u
A mischievous child is called unruly. It begins with the letter u.
minors