Rather than trying to get the kids reading the adult shorts and novels try the books they wrote for kids
Asimov has the Luckey Starr series. This was probably the first Sci-fi book I remember my dad reading to me.
David Starr, Space Ranger (1952)
Lucky Starr and the Pirates of the Asteroids (1953)
Lucky Starr and the Oceans of Venus (1954)
Lucky Starr and the Big Sun of Mercury (1956)
Lucky Starr and the Moons of Jupiter (1957)
Lucky Starr and the Rings of Saturn (1958)
From Heinlein
Rocket Ship Galileo, 1947
Space Cadet, 1948
Red Planet, 1949
Between Planets, 1951
The Rolling Stones aka Space Family Stone, 1952
Farmer in the Sky, 1953
Starman Jones, 1953
The Star Beast, 1954
Tunnel in the Sky, 1955
Time for the Stars, 1956
Citizen of the Galaxy, 1957
Have Space Suit—Will Travel, 1958