This was Robert Heinlein's first juvenile novel, published in 1947. In its alternate post-WW2 timeline, the United Nations (and, in theory, only the United Nations) holds nuclear weapons "for peacekeeping purposes". Rocket travel is commonplace, but apparently only suborbital. As the book opens, three young boys (Art, Morrie, and Ross) are testing their own rocket in a remote field. It blows up, but never mind that: on their way home, they discover a bloodied unconscious man and it turns out it's Art's uncle, famous Manhattan Project physicist Dr. Donald Cargraves!
He recovers quickly, only to enlist the lads in an audacious scheme: to use his mad nuclear skillz in a rocket that will take them to the Moon! After some parental haggling, it's on: off to the desert to build, test, and launch Cargraves' unique design. They are working "on a shoestring" budget, buying an older freight rocket at scrap prices, using Cargraves' connections to get fissionable material, and after a few months of welding, they're off. There have been some intrusions and sabotage along the way, but they make it.
Only to run into (spoiler ahead) Moon Nazis. Apparently they figured out the same thing Cargraves did, only slightly quicker. (I guess Wernher von Braun stuck with the Nazis in this alternate future.) And their nefarious plans involve raining down nuclear destruction on Earth. As soon as they get rid of these pesky intruding Americans.
Well, it's all pretty ludicrous. The Nazi stuff is cartoonish. But in a fun way. Cargraves is the classic Heinlein Sage, many pages devoted to his semi-cantankerous dispensing of knowledge, philosophy, and opinion to the kids.