I first read "The Master" as a pre-teen and have remembered it for decades. I recently was reminded of the book and wanted to read it again.
Forty years later, it's still good. The children act like children, an adult reader can see through to some of the deeper plot lines, the characters are interesting and engaging. It's not as whimsical or cute as White's "Sword in the Stone" but it's got some of the same flavor.
I did note that "The Master"'s plan for taking over the world is a pretty silly one--there are faster, more secure ways to do that given his mental powers. See Azimov's "Foundation" series for how "The Mule" uses a similar power in a more sensible way.