Little Men of Space — Frank Belknap Long

When two children discover three tiny alien visitors emerge from their father's experimental rocket, they are thrust into a terrifying encounter. The ruthless leader of the aliens plots to hijack the rocket back to his home planet, but the children discover they can read the thoughts of the aliens. A battle of wills ensues between the children and the aliens over the fate of the rocket, with the safety of the children and potentially the planet at stake. Can the children outsmart beings from an advanced race to save themselves and humanity?