Understanding digital supports for early learning is paramount for school readiness and later mathematics learning. We present results from a randomized control trial evaluating a digital app (Measure Up!) and a parent companion app (Super Vision) designed to teach children measurement concepts, a skill that many teachers do not feel comfortable teaching. Ninety-nine 4- and 5-year-old children were randomly assigned to one of three conditions: Measure Up!, Super Vision + Measure Up!, or a control game. Analyses revealed a statistically significant effect of being in the treatment group (Measure Up! or Measure Up! + Super Vision) on children's posttest scores (about two additional questions correct), controlling for the pretest and demographic characteristics (gender, SES). In particular, gains were made for children's understanding of pan balances. There was no significant difference between the two treatment groups. Results suggest that apps can be designed to help children learn important mathematics skills; however, more research needs to be done to understand how parent supports can be better designed. Implications for evaluation and design of game-based learning tools are discussed. [This paper was published in "Computers and Education" v146 2020.]