L.R.
Does it just have to be wooden? Ikea sells excellent plastic step stools that have non-slip bottoms as well as non-slip "dots" on the top surface so kids don't slip. They are very durable and everyone around here has one. My daughter is 10 and we all use hers all the time including my husband and me -- to reach stuff in cabinets, for her to rest her feet on when playing the piano, as a floor-level small table, etc. They are one step, not two, but do the job very well indeed and last forever. And they are solid, not folding. I agree that any folding step is likelier to break or collapse. Try Ikea.