Stowaway to the mushroom planet by Eleanor Cameron 1956 Scholastic
No other Earthlings know the Mushroom Planet like David and Chuck. They have seen the green ball shining in the vast black of outer space. And they have walked in its giant mushroom forests.
Now they are going back. The flight is top secret. But someone's found out, and stowed away on the space ship! How did he sneak on board? And how can Chuck and David save the Mushroom People from his dangerous plans?
