We report the effectiveness of using the tablet-game Kalulu Phonics immediately after intervention in kindergarten and on national evaluations the year after. In a previous intervention testing the software with 1st graders, 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 the matched Kalulu Numbers control game for the first half of the year, with reversed assignments in the second half. 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 correspondence (GPC) matching and fluency. Students with the number version improved in number knowledge. In a longitudinal follow-up, all intervention participants maintained an advantage in phoneme awareness and GPC matching at the start of 1st grade, but this advantage failed to translate into school literacy gains by mid-year. No longitudinal benefits were found for numbers. Our results support using tablet-based aids in spurring early reading skills but question the possibility that a short-term intervention may address the challenges of long-term educational goals.