There must be 50 answers. Here are a couple off the top of my head:
--Humor is the most obvious thing.
--Accessible language. Some of the dialect may be difficult and of course
some of the lingo is dated. But by and large Twain is crystal clear.
--Good stories. He's a gripping yarn spinner. Never forget that the two
most famous "kids' books" are titled "The Adventures of...." and
--And here's one more special trait that has always struck me: In Tom
Sawyer and Huckleberry Finn Twain strikes almost the perfect balance for
kids between reality and wish fulfillment. It's perhaps more obvious in
the earlier book: Tom gets the girl, gets rich, runs away and gets away
with it, attends his own funeral, bosses the other kids around, etc., etc.,
etc. All those wonderful things a kid might dream of doing.....
BUT Tom also takes his lumps. He's a very real child as he squirms through
an endless school day or a Sunday sermon, is forced to wash, to wear nasty
clothes. He feels guilt and a good deal of shame, and fear (even
terror). He makes a fool of himself... and that list too could be greatly
It's a truly wondrous blend of "Tom is the kid I would love to be!" and
"Tom is like me!"