"The Incomplete Amorist" by E. Nesbit

8-year-old Betty, the respectable step-daughter of a Vicar is surrounded by moral and religious culture. Her world becomes surprisingly overturned when she meets Vernon, an artist, ten years her senior, without a chaperone. She sees the meetings as educational as he's a serial womanizer. However, when you meet someone who makes your heart flutter, it's impossible to give them up.