There was a children's book I used to have which I believe was about two kids who go to space and the moon (I think it's the moon) with an old guy (maybe a grandpa or just a crazy looking scientist) they explore the moon, one page had a circular metallic spot, I can't remember if it was a mirror like material. I also remember there was an alien but not sure if the characters were aware, it might have just been a background picture. It was a hardcover book and not more than 20-25 pages. Probably from the early 90s or older
The Wonderful Flight to the Mushroom Planet (Eleanor Cameron, 1954) comes to mind: two boys, one rather fussy scientist (who stays behind on Earth), and lots of mushrooms. The cover has a bright circle representing the Earth that is differently colored in different editions. But the book is considerably longer than the one you describe.