The Kalulu software is a tablet-based suite of phonics and reading-related lessons and minigames. In a previous intervention with a previous version of the software in 1st grade students, fluency and comprehension were boosted, but only when used in concert with reading instruction at the start of the year. Here, we asked whether a similar intervention would be more efficient if it started a year earlier, in kindergarten. Forty classes (1092= children) were randomized into playing Kalulu phonics or an active matched control game (Kalulu numbers) for the first half of the year. Those assignments were reversed in the second half of the year. Ten non-randomized business-as-usual classes also participated. In a cross- over effect, children who used the phonics version improved in letter naming, grapheme- phoneme matching and reading fluency, while those with the number version improved in number knowledge. In a longitudinal follow-up, intervention participants maintained an advantage in phoneme awareness and grapheme-phoneme matching at the start of 1st grade, but this advantage failed to translate into school literacy gains in the middle of 1st grade, and no longitudinal benefits were found for numbers. Those results improve our understanding of when and for how long to introduce phonics and question the possibility that a short-term intervention may address the complex challenges of long-term educational goals.