The core group are the twelve novels published by Scribner's between 1947 and 1958. The series started with some fairly minor work, but by its end included novels that stand up as well or better than his adult novels.
A thirteenth, Starship Troopers, was submitted to Scribner's but rejected and instead published by Putnam. A fourteenth novel, Podkayne of Mars, is often listed as a "Heinlein juvenile," although Heinlein himself did not consider it to be one.
Many fans consider the Juveniles to be the high point of RAH's writing. While there's no doubt that his earlier incarnation (roughly 1939 to 1950) as a primarily short fiction writer was more influential on the field, the Heinlein Juveniles, and especially the later ones like Citizen of the Galaxy and Have Spacesuit, Will Travel, were the first taste of first-rate SF for a great many fans who came of age in the 50s and 60s. These books shaped an entire generation.
Heinlein's Children is an interesting collection of essays about them.