As part of a Master project, I used the Manulex database [1] to compute statistics on the vocabulary known for a child of a given age. The goal: Calculate the percentage of infrequent words (i.e. that does not belong to the means vocabulary of a child of a given age).
[1] Lété, B., Sprenger-Charolles, L., & Colé, P. (2004). Manulex: A grade-level lexical database from French elementary-school readers. Behavior Research Methods, Instruments, & Computers, 36, 156-166.