In keeping with my kid's stuff made from 12mm Baltic Birch, I drew up the plans and cut out some of these this week. My son will soon need a way to get up to the sink (I am hoping!).
The treads are curved on the front, and the treads and risers are both set into a rebate in the sides, as well as the riser fits into a grove in the bottom of the tread.
I cut them out with a few small tabs, and it worked great. I can get 3 units from a 48" by 48" sheet of ply, with minimal waste.
If anyone wants the file I will upload it here later.
This one will be painted, so dont mind the little bit of glue squeeze out I didnt remove
AJC