My daughter refused to use a kiddie potty from the get-go. For her, it was all about straddling the "big girl" toilet (and holding on for dear life). I guess that's the most eco way to go, though lots of little kids actually like having their own personal potty (and lots of moms like not having to worry about their child falling in the toilet every time they use it).
For those kids and moms, here's a great eco-friendly potty training option: The Beco Potty.
Made of bamboo and rice husks, Beco Potties are ergonomically designed for your child's comfort. The potty is biodegradable, which means when your toddler is finished potty training, he can plant it in the garden.
Seriously.
Beco Things recommends you drill a few holes in the bottom to let the moisture through. Then submerge the potty in the dirt, add some seeds and a little water, and watch something grow. The potty will biodegrade over the next few years, feeding your flowers and helping the environment.
$12.99 at Amazon.