You can buy tickets for three shows (abonnement in french), it gets you a reduction that's equal to the tarif jeune.
The cité de la musique is a classical music oriented auditorium, 2000 seats i guess, absolutely child friendly.
The cabaret sauvage, i've never been there, but can tell you it has a circular stage, is non smoker like all venues in france. It's also 5 minutes walk from the cité de la musique.
http://www.cite-musique.fr/francais/billetterie/abo_ligne_jazz_0809.asp
Here is the page to get an 3 shows formula with better price, you just multiply the number of people at the bottom of the page. I wouldnt count on getting reduced tickets at the door in this festival, it's probably gonna be sold out.