That is the The Mushroom Planet series by Eleanor Cameron. From the first book, The Wonderful Flight to the Mushroom Planet:
When two boys find an ad in a newspaper asking for two young boys to build a spaceship, they quickly construct one out of old tin and scrap wood, and bring it to the advertiser. This man is the mysterious Mr. Tyco Bass, an inventor and scientist. Using his marvelous stroboscopic polarizing filter he shows the boys a previously undetected satellite of the Earth, which he calls Basidium-X. He refits their spaceship, giving them some special fuel he invented to power it, and tells them to fly to the mushroom planet (after getting their parents' permission). He warns them that their trip will only be successful if they bring a mascot.
When it is time for launch, they grab David's hen, Mrs. Pennyfeather, at the last moment for a mascot, and rocket into space. They find the planet of Basidium to be a small, verdant world covered in soft moss and tree-size mushrooms. They quickly meet some residents of the mushroom planet, small men with large heads and slightly green skin, of the same people as the mysterious Mr. Bass. They tell the boys that their planet has had a crisis and that everyone is slowly dying of a mysterious sickness. The boys meet up with the king of the planet, the Great Ta, and end up solving the natives' problem, before returning to Earth.