1.
First idea involves three princesses and a bigger royal family/court. An older one, probably around 15. A younger one, who is twelve (and may have been adopted into the family)....and an even younger one age 9. The oldest has an attitude, but is also the smartest and most mature one naturally. The youngest one is cheeky and a troublemaker. The middle one is the least troublesome and a bit "coddled".
I imagine the queen, or a consort is responsible for disciplin. But the king or a brother might get involved too.
Its basicly a medieval setting. I imagine it takes place somewhere on the British Isles, or a Fantasy version thereof. Its an idyllic kingdom of great warriors, bards and culture.
2.
Another idea is less developed. Basicly its something in dark age europe or the middle east (so early medieval). Everything else is basicly not settled, except I would prefer a girl spankee.
3. I forgot idea 3
4. A girl (10-11?) takes something from her parents and is spanked by both. Either at the same time or one after the other. Maybe one is the "scary" parent.
5. Twins...one being the more troublesome one. Maybe they go stir up a hornets nest or something.
Anyway, please contact me and tell me if you want to suggest something.