Dorilu.net is a website where you can customize and print various drills for learning in Japanese (all free!) . There are drills for math, Japanese language (mostly kanji practice), geography, and English. You can even search for drills by grade level (1st~6th grade).
Looking at these drills makes me nervous for my daughter to enter elementary school. Some of these worksheets are very difficult!
from dorilu.net
Hiragana Mama 
Ack! This is perfect too! How did I miss this?! My first grader is already working on kanji (they’ve learned 16 only 4 months into school!), and I’m working with him so he’s always ahead of the class…. Gotta do what you can when your kid starts from behind :-). Anyway worksheets for his level and beyond are fantastic! We’re at the mall getting kumon workbooks all the time, its great to have options from home (we used the bother katakana stuff you linked to before.) Thanks again!