Now that's an answer! Thank you Mac! I like your motto!Adult size chassis. Reason is, if he/she likes sprint and wants to continue racing sprint you'll need one for whichever class they chose for racing with adults. Height isn't a factor as there's pedal extensions and your steering shaft can be brought down lower to accommodate for that as well.
Not to mention having an adult chassis will be easier to sell to the local club than a jr or kid kart if you get out of it.