Nanny comes back from the village early and catches them in a compromising position. Instead of throwing the teacher out and sending Wendy to her room at once her anger explodes and she turns Wendy over her knee and spanks her in front of him while lecturing them about their behavior.
Somewhere in the middle of this the teacher and Wendy seduce her. 
From here the story could branch several directions: 1. Nanny's hidden passion causes her to completely surrender to them both. 
2. There is a short lesbian scene and when the teacher joins in they both service him. 
3. Nanny's dominant nature could lead the story in many interesting directions. 
In the end you could even have Nanny's guilt cause her to say she has no right to correct Wendy if she is allowed to get away with such behavior herself and say she needs to be punished. When Wendy says she doesn't want to hurt her Nanny would say she has to quit if she doesn't punish her. This could lead to a session with Wendy after the teacher leaves.
You could even have Wendy's father find out about the Afternoon and after dealing with Wendy, either in the game or as a reference, Nanny could be faced with trying to persuade him to let her keep her job.
This story could lead to part 2, part 3, even part 4. Just some ideas. Happy gaming.
