I found this in a copy of Super Friends #5 (June 1977).
Don't know who Colleen Doran is? Well, check this out, yo.
This letter was previously discussed in a blog entry at Gorilla Daze, but that was something I didn't know when I scanned it. The interesting thing is that, according to Doran herself, she was born July 24, 1968 (though some sources erroneously list it as 1963). This particular comic went on sale March 21, 1977, which means her letter was likely written sometime in 1976. When she was 8.
Go back and read it again. Not bad for an 8 year old, huh?