Wonderland in USJ featuring snoopy, sesame street and hello kitty is very suitable for kids of 4 and 6 years old. Check it out. My girls stayed in wonderland for the whole day! No need to buy fast pass as it is not crowded in wonderland. There are free fast pass for the rides in wonderland.
Wonderland games
http://www.usj.co.jp/e/attraction/uwl/#03
Flying snoopy, Elmo boat requires 92cm min. Others like 放鴨仔,driving school (3 years and up ok), Cookie Monster slides kitty cupcake, and many others all ok