When I was 7 my two best friends, one a boy and one a girl, spent hours in the library poring over anatomy books because we thought it was thrilling to look at picutures of naked adults (sans skin, of course).
I'm not sure exaclty why oral sex needs to be in the dictionary. I mean oral is in there, and sex, so why on earth do we need to include PHRASES as well as words?
Other than that, put the dicitionary up with the other books that kids get to look at but not check out, and monitor them while they're using them. If they're drawing pictures and giggling, it's probably time to look over their shoulders.