A.H.
I bought a new toilet seat that has a toddler sized opening that can lay down over the adult sized opening. I got mine from Walmart for about $25. It's worked out really well. My son uses a stool to rest his feet on and he did really well with it. He's getting old enough now that he doesn't need it and he uses the adult seat more often than not, so I'll be trading it out for the regular adult seat soon.
http://www.amazon.com/Magnolia-Progressions-Adult-Toddler...