This is the Mushroom Planet series by Eleanor Cameron.
The description from the Wikipedia article is:
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.