oooh, this is very cool, and totally in context for my blog too. I may have to join you! I don't have any pets left lower than lvl 10, so that could be an issue, haha.
From my observations thus far (see this post:
http://tamerliopleurodon.wordpress.com/ ... -and-pets/ ) this will get a lot less awful when you get into the teens, and progressively less awful as you get higher with a cap at level 85, because you can hang out in lower level zones and still get full XP... I didn't SS it, but my level 83 DK was getting full XP from random pets in Desolace. The bottom is really relatively low. You'll level those super low level pets really fast though... you may need to buy & sell a few clowders of cats to level your character out of Elwynn/Durotar. The XP leap from 85 to 86 is really huge, which is why it'll likely go from not too bad to terrible again, but you can hang out in Icecrown and hunt whelps or whatever.
Good luck, and congrats on a super interesting project!