Louise,
I tried tp rolls too, but they are to soft and would get squished a bit. If you get one of those stools, just get one of those ones with two steps as they are higher.
If you look on the squatty potty website, they give you an idea of how tall you need depending on how tall your toilet is. The squatty potty stool itself wouldn't make a good stool for a toddler though, as it is on a slight angle.
http://www.thesquattypotty.com/
-Tanya