bananas, I advice you to read up on navel piercings before you get it done; they're one of the HARDEST piercings to get to heal properly (it takes 6 months before it's healed).

I used to work as a piercer, and every day I'd have at least 10 15-16 year olds come into the studio, pull their top up and say "my navel hurts! it's red! Help please!". Half of the cases were because they either pierced it themselves (or by a friend), or got it pierced in a hair salon - ugh.
The other cases were because they didn't care about following the directions, so they'd go swimming, change the barbel themselves (and usually add really heavy jewelry), did sit-ups or other exercises for the stomach or back, didn't rinse properly, and generally the navel wouldn't be kept as clean as it should.

So please, please - follow the directions religiously. You will get some serious scarring, and in worse case scenario - keloids, if you don't. You don't want that, trust me.

To answer your question; yes, it should be possible to get it done with a parent/legal guardian present. It's that way here anyway, but if the client was younger than 15 (age limit for normal piercings is 16) we'd have to evaluate it on a personal level since they'd be more likely to mess up the healing process.

PM me if you have any questions btw.