This sounds very much like *Rusty's Space Ship* By Evelyn Shipley Lampman, 1957.

Two children, Rusty and Susan, build a play spaceship, which becomes a real spaceship when they get involved with a lizard like alien who visited Earth and now wants to return to his home planet Eopee. 

It is mentioned in various previous answers:

https://scifi.stackexchange.com/questions/202126/kids-book-about-boys-finding-a-disc-from-a-planet-called-eyopee/202129#202129[1]

https://scifi.stackexchange.com/questions/214382/60s-book-about-a-boy-who-builds-a-spaceship-from-cardboard-in-garage/214384#214384[2]

https://scifi.stackexchange.com/questions/66578/childrens-novel-three-children-plus-dog-travel-to-the-moon-in-pretend-space/145518#145518[3]




  [1]: https://scifi.stackexchange.com/questions/202126/kids-book-about-boys-finding-a-disc-from-a-planet-called-eyopee/202129#202129
  [2]: https://scifi.stackexchange.com/questions/214382/60s-book-about-a-boy-who-builds-a-spaceship-from-cardboard-in-garage/214384#214384
  [3]: https://scifi.stackexchange.com/questions/66578/childrens-novel-three-children-plus-dog-travel-to-the-moon-in-pretend-space/145518#145518