Grimm's Fairy Tales, Bugs Bunny, the whole vampire fascination...those are dark, too and kids to young adults love them. We love to be scared!
Just because you want to write for kids doesn't mean you have to eliminate your darker thoughts. You may want to temper them with comedy or lighter moments in between--maybe have one character who is especially Eeyore, a couple of Winney the Pooh and Tigers for balance.
Don't throw away your success, just expand. Work outward from that dark side and add some color and light.