Perhaps the novel "Kidnappers of Space" released in the U.S. as "Space Captives of the Golden Men" by M.E. Patchett in 1953.
Points that match:
I am looking for a juvenile scifi published no later than 1961
Match. from 1953
(when I read it) about two boys--aged 13 and 15--traveling to the far side of the moon.
Match. The year was 1976 and the great new rocket desgined to carry the first men to the Moon, would soon be launched. But Jim and Bob Steel, sons of the Interplanetary Society in charge of the project, were destined to visit the Moon before the rocket !
Their father is part of a planned mission to the same destination, but they boys are taken there first by very tall aliens (abducted by them?) who get around their ship by means of some kind of tubes that the boys get used to using.
Match. They were kidnapped by the golden men of Mars, crash landed on the Moon, and were the first humans to explore its mysteries.
Their father and his moon mission had seen photos of the far side of the moon in which they had seen some kind of structures that looked above like frog spawn. The boys somehow explore one of those structures and find another alien race that is living there but tyrannized by another monster.
Match. They were the happy race of little troglodytes, discovered the horrible wraith-like ''Zombies'', and came face to face with the Wormisaur.
The boys are befriended by one of the tyrannized moon dwellers;
I am pretty sure his name is Gik, since part of the plot has to do with the difficulties of second language learning.
I particularly remember that Gik makes swimming gestures when faced with a body of liquid and asks, "What is this?" or something, they reply "Swimming," so Gik says, "Swimming after Gik." The boys and the native race are able to defeat the bad tyrant with some kind of bomb.
Match. They blow up the Wormisaur with one of the Golden Men from Mars explosives.
I was reading all the juvenile scifi that our branch library had. I would have remembered if the book were either by Heinlein or Clarke; it was not. I believe there were touches of the current space race in it, so it could not have been written before the 50s. I believe the library binding was red, but that might not be much help. I have been trying for years to locate this book again. I read it over and over and over.
Match. The rocket lore, the interplanetary flavour of the book, and the astronautical information are as accurate as present knowledge allows. M. E. Patchett is well-known for imaginative and vivid writing and this exciting book is one of the author's best. Here is a story which all readers will enjoy for it gives a glimpse into the future - perhaps their future as explorers of the Moon. Bookseller Inventory # 1086
A description of the book is listed at the following site:
Cover title (one of a number so if does not match what you remember, may not be incorrect_
True First Edition. From copyright page: "First Published 1953" The book's boards are moderately worn, the endpapers and edges of the pages are foxed, otherwise free of inscription, tightly bound and in very good condition. (VG/No Jacket.) 208pp. This is written under the initials M.E, obviously the old fashioned notion that a female author does not appeal to boys. It lists Ajax and the Drovers under author's other books. A scarce title indeed, a wonderfully unusual copy of her work. The year was 1976 and the great new rocket designed to carry the first men to the Moon, would soon be launched. But Jim and Bob Steel, sons of the Interplanetary Society in charge of the project, were destined to visit the Moon before the rocket ! They were kidnapped by the golden men of Mars, crash landed on the Moon, and were the first humans to explore its mysteries. They were the happy race of little troglodytes, discovered the horrible wraith-like ''Zombies'', and came face to face with the Wormisaur. This thrilling story is not just a fantasy.