This paper deals with the effectiveness of summer learning programs wherein parents play a particular role in creating a learning environment at home. A fairly simple and low-cost evidence-based field experiment is set-up in order to estimate the effectiveness of summer learning for children who prepare their selves in kindergarten for the transition to the first grade of Dutch primary compulsory education. Children assigned to the intervention group received a summer book with exercises dealing with math and language. They could practice in the summer book for about 6 weeks. Parents were explicitly asked to assist their child with troublesome exercises. And teachers did assess children's educational proficiency before the start of the summer break. Standardized test scores of the treated and untreated children have been measured before and after the intervention. Our findings show that language proficiency increased significantly (+5.5 points) in the treated group compared to the untreated group. We did not find a significant effect on math proficiency. It is also argued from the results that using teachers' assessment provides better statistical modeling than using pre-test scores.