I, like you, am most comfortable having some published material to draw from that gives me a framework for my campaigns. Sadly, there doesn't really seem to be much yet. However, just read the book. There are a lot of ideas in there, especially when reading about the places and people of Grimm. My plotline began to create itself as I read along - the children trying to find "Babylon," a rumoured way to get home. I decided on 12 keys, hidden across the land that they would have to collect, which is a great focus for the players and allows me to make them have to go to lots of places looking for them. I also decided that early on they would encounter the minions of the Rotten King who will hound their every moment, chasing them as they search - which gives the story it's impetus.
Who they meet and where they will have to go I will iron out on the journey. Everything you need is there. Just remember, anything ever imagined can exist somewhere in Grimm. What stories do you love that you want to put the children through? I love ALice in Wonderland, so you can bet they will encounter a grinning cat, playing-card guards, evil head-chopping queens and maybe even the Liddell girl herself - a la American McGee's butcher knife weilding brat, I'm sure.
Maybe they'll hear the Smurf song and investigate, only to have Papa Smurf command "Attack!!" and dozens of blade-weilding blue bastards start swarming over the kids. Perhaps Flipper will help them when they reach the sea. Maybe the Far-Out Space Nutz will in to refuel.
You are up to it. Let your imagination wander