Rascal.
mischievous
mischievous sprite
A mischievous child is called unruly. It begins with the letter u.
Rascal.
imp
jackanapes
mischievous
imp
tyke
deviltry
Rascal might be the word you are looking for.
The word "ragamuffin" refers to a child that is shabbily dressed. The word "urchin" is also often used to refer to a mischievous child who is raggedly dressed.