Since the audience is young children, the premise should be kept fairly simple, and be something that they can relate to and understand.

"Dexon my friemd, you have come to help. Very good." hisses the bright green dragon. "Someone has stolen the city's treasure. We must find them and recover the treasure, or the city people will not have money to buy food."

Pointing up the road the dragon continues, "It looks like the sneaky thieves were in a hurry, and were not very careful." Gesturing towards a shiny gem, the dragon adds, "They have left a trail of magical gems scattered along the path. Gather them up to improve your strength. Follow the trail of gems and find out who has taken the treasure. We must return it to the towns people quickly."

"Along the way we will encounter many challenges. I will help and guide you when I can, but I must keep myself hidden. Many other knights are looking for dragons to slay. They don't know that we are friendly. When you need me, just whistle by pressing the "?" key on the keyboard. I won't be far away. You may keep all of the weapons and special items that you find during our quest, and the mayor is offering a fine reward."

Are you ready, young knight? Then let us begin!

Cheers!
Mark