Piotr Wozniak, inventor and author of spaced repetition algorithms (invented the SM-2 algorithm used by Anki around 1986), states quite clearly in his research that these algorithms are not designed for children under about 7 (of course super high IQ children's brains are different).

He reviewed a vast amount of performance data to conclude that, and hypothesized that children's brain before that age is not designed for remembering vast amounts of data, but instead learn through exploration and other natural, non-academic means.