Information on the number of prescriptions written to minors for anti-depressants can be found in Health, United States 2006 at Table 92. (located on page 331 of a very large .pdf file).
Briefly in the period 1995-96 there were 1.9 prescriptions written for anti-depressants for every 100 persons aged 18 and under. By 2003-04 that number was 8.0. While both boys and girls received anti-depressants at the same rate (1.9) in 1995-96, in the 2003-04 period boys received 9.1 prescriptions per 100 persons and girls received 6.8.
Unfortunately, we did not collect this data before 1995.