We evaluate JumpStart, a parent-directed school-readiness program run by a Houston-area school district, in a randomized trial spanning three cohorts. Being offered the program raises scores on a curriculum-aligned assessment by 7.6 percentage points and parental investment by 0.26 standard deviations. A model of skill formation with endogenous inputs and program participation yields an average treatment effect of completing the program of about 9 percentage points, and attributes roughly a quarter of this effect to higher measured investment. Nonparametric bounds corroborate the average effect, and a mediation analysis that relaxes part of the model's structure supports the decomposition.