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Thought about this book too on occasion, but never tried to find it before now. I did a quick Google search and came across this Amazon page
, which seems to support the idea that it was a book series. The editorial reviews mention the cogs, but in a context as if they were always part of the crew, instead of being newly discovered.
From there, I searched for "Jed's Junior Space Patrol". This page
has an animated GIF that flips through the pages of the book, but they're too small and too fast to really be intelligible. However, I caught pictures of a cave, a boulder, and two fuzzy creatures, so it's almost certainly what we're looking for.

I'm looking for a kid's book that I read once in a library. It was about a little boy who goes to Mars (or possibly the Moon) in a rocketship. I don't remember why he went, or if he had anyone with him. While he was there, he encountered these adorable fluffy creatures who could talk and explained that since they were part cat and part dog, they were called "cogs." I think there were two baby cogs, and their mother might possibly have been killed in a rockslide? I don't remember what the rest of the book is about, just the one scene.
There were illustrations, but they weren't in color. They were like pencil sketches, sort of like the illustrations Edward Gorey does but not exactly.