There’s a book I read around 2004, definitely before 2005. Man travels Europe with his rich girlfriend, they have no real jobs. Her preteen daughter is home from boarding school for the summer so is traveling with them. At the beginning of the book she uses yogurt to soothe a sunburn. Also, he wakes up to the child standing next to the bed asking him to take her on pogo sticks but he refuses. For some reason they have to go visit the child’s grandfather. The child helps the main character plan to steal some art while at the house. He is a little bit bumbling but succeeds at stealing the painting. While they are at the grandfather’s house he has to pretend to be a tutor for the child. There were other people also there, and one was a French (?) woman who flirted with him a lot. At the end of the book the man takes the child on pogo sticks.