Uruguay has increased its preschool enrollment, reaching almost universal coverage among four- and five-year-olds. However, more than a third of children enrolled in preschool programs have insufficient attendance, with absenteeism higher in schools in lower socioeconomic areas and among younger preschool children. This paper presents the results of a behavioral intervention to increase preschool attendance nationwide. Most previous experiments using behavioral sciences have looked at the impact of nudging parents on attendance and learning for school-age children; this is the first experiment looking at both attendance and child development for preschool children. It is also the first behavioral intervention to use a government mobile app to send messages to parents of preschool children. The intervention had no average treatment effect on attendance, but results ranged widely across groups. Attendance by children in the 25th 75th percentiles of absenteeism rose by 0.320.68 days over the course of the 13-week intervention, and attendance among children in remote areas increased by 1.48 days. Among all children in the study, the intervention also increased language development by 0.10 standard deviations, an impact similar to that of very labor-intensive programs, such as home visits. The intervention had stronger effects on children in the remote provinces of Uruguay, increasing various domains of child development by about 0.33 to 0.37 standard deviations. Behavioral interventions seeking to reduce absenteeism and raise test scores usually nudge parents on both the importance of attendance and ways to improve child development.