The transition from informal to formal mathematics is an important episode in children's mathematical development. The current study investigated how young children's computational estimation performance and strategy use develops in this transitional period. The computational estimation performance of 350 children was assessed before the start of formal schooling (i.e., third grade of kindergarten) and again after the start of formal schooling (i.e., first grade of primary school) by means of a computational estimation addition task with manipulatives. Both children's answer construction and their counting behavior while constructing the answer were observed during task administration. Results showed an age-related increase in children's estimation accuracy as well as in their proportion of exact answers. Age-related changes in strategy use were also observed. Children demonstrated an increase in their counting behavior while constructing the answer, but no changes in the way the answer was constructed. In both grades, the answer was most often constructed by laying down all manipulatives immediately in one group. These results suggested that children can follow two pathways to solve the estimation problems: (1) relying on the visual representation of the addends without using counting and (2) using the verbal labels provided by the experimenters while using counting. More use of counting in first grade positively influenced children's estimation accuracy in this grade, suggesting that these children strive for more precision compared to children who do not count.