Mary Pope Osborne 
Magic Tree House 8: Moon Mission! 

Eight-year-old Jack and his little sister, Annie, are playing in the woods during their summer holiday, when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . .
Jack and Annie are whisked into the future — to an exciting outer-space hotel! But they»ve only got two hours before they run out of air . . . Will the mysterious moon man and his map help them get back safely?