Meriol Trevor’s 1957 The Other Side of the Moon is a standalone juvenile SF novel.
The Kingville Lunar Expedition did not intend to take an untrained boy on their mission to the Moon. Nor did Gil Pentfoyle intend to stow away. Having snuck on board to play spaceman, Gil fell asleep. The busy crew overlooked the boy until the expedition was well on its way to Earth’s satellite.
Although displeased to discover the stowaway, Captain Raider doesn’t airlock Gil. Perhaps this is because Gil’s brother, the expedition botanist1, might object. Perhaps this is because the captain is decent guy. Besides … the expedition already has a useless member, the expedition sponsor’s son Tracy. Gil makes two of them. (Perhaps three; see1.)
Turns out that the Moon is a surprising place.