Youth. Isaac Asimov

Two children chance upon a mysterious creature and soon find out that it is none other than an alien from another planet! Hiding it from their parents and trying to keep him safe, will the children be able to help him go back home? Excerpt: Red kept to his croaking whisper, «Quiet! You want to wake somebody?» Slim noticed all at once that the sun scarcely topped the low hills in the east, that the shadows were long and soft, and that the grass was wet. Slim said, more softly, «What's the matter?» Red only waved for him to come out. Slim dressed quickly, gladly confining his morning wash to the momentary sprinkle of a little lukewarm water. He let the air dry the exposed portions of his body as he ran out, while bare skin grew wet against the dewy grass. Red said, «You've got to be quiet. If Mom wakes up or Dad or your Dad or even any of the hands then it'll be 'Come on in or you'll catch your death of cold.'»