Not only children but also adolescents and adults encounter great difficulties in learning to reason proportionally. Despite these difficulties, research increasingly shows that proportional reasoning emerges early, before it is being instructed in school. There have however been very few attempts to stimulate this early emerging ability. The aim of the present study was to stimulate proportional reasoning in second graders. We developed an intervention program focusing on quantitative reasoning and promoting different strategies to solve proportional missing-value problems. The effectiveness of the program was evaluated in a pretest-intervention-posttest study with a control group (n = 139). Results showed a large effect of the intervention program on children's proportional reasoning abilities in fair-sharing situations and a small transfer effect to word problem solving. There was also a moderate effect on the proportional vocabulary that was explicitly taught in the intervention program, but no transfer effect to proportional vocabulary not explicitly taught.