When my two year old was in his first year, we used to listen to a lot of jazz (we still do). He loved it!
Some of his favorites were:
Flip Phillips - Best of the Verve Years
Lee Konitz - Motion
Johnny Hodges
He really seemed to like stuff of the small group variety. If there was too much going on, he didn't get into it. He also really likes organ jazz. He loves Lou Donaldson's early sixties stuff, like the Natural Soul.
And... he digs classical too, not symphonies, but string quartets. Lately, I've been playing a lot of Haydn and Handel for him.