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
"Adventures of....."

--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
expanded.

It's a truly wondrous blend of "Tom is the kid I would love to be!" and
"Tom is like me!"

Mark Coburn