Shenanigans
impish
hobgoblin
Goblin
imp
Rascal might be the word you are looking for.
Imp.
heathens, hellions
Jocular?
the letter is u
mischievous
Idle, idol, inch, inky, itch, iced, iron...
A mischievous child is called unruly. It begins with the letter u.
imp
hellions
pixie
Rascal.
Brat.