While my children start on a child sized potty, they both insisted on transitioning to the adult sized toilet shortly after becoming comfortable with the process. Over the last few years, I have tried various arrangements of toilet seat adapters and step stools to properly accommodate them. I think I have finally stumbled across a workable solution.
We replaced our toilet seat with the NextStep Built In Potty Seat. I love this product because the child sized seat is attached to the adult sized seat and nests inside the lid. I also like the child seat is nearly identical to the adult seat in shape and material. Both Elora and William have taken to it exceptionally well. I am thankful for the friends who recommended it.
The specialized step stool is called the Looster Booster. I have found it to be far superior to traditional step stools for climbing onto the toilet. The wide base has given them both confidence. Additionally, it gives them support for their legs since they are able to rest both feet on it. That is often a critical piece for correct toileting behaviors as dangling legs can affect the ability to use the necessary muscles correctly. The platform is wide enough for William to stand on since he prefers to use the restroom "like daddy" now.
Our setup has been so successful that I adapted the second toilet the kids use in an identical fashion.