DD is used to and loves sitting on the big toilet but even with a step she cannot get herself up there by herself. She can climb on but not sort of twist herself around the open whole to get into position. I didn't want to use a potty though.
my ds is similar with the step. he can do it, but it is awkward and in a rush it's no good.

I think my dd just ditched the ladder/seat early and used a step and perched on the seat. I saw great inbuilt toddler seat at Bunnings but it didn't fit our loo.