Answer: Longitudinal study
Explanation: Longitudinal study refers to recurrent observation gathered on a particular set of variables over a certain period of time. Longitudinal studies could be carried out on the same variable or set of variables over a short or long period of time ranging from days, weeks, years or even decades. They are mostly conducted to study changes in traits of measured variables over time.
Testing memories of a single group of children over a constant period of time could be used to determine variation in memories of children as they age.