While it's probably much less common, I've never even heard of a woman getting arrested for something sexual with a pre-pubescent child. A few scattered cases of women with teen boys - and they get a slap on the wrist.
I forget where I read it, but I remember an anecdote where someone was at dinner at a restaurant with a bunch of other people. A women had a girl, maybe her daughter on her lap and kept rubbing her crotch through her clothes. No one even said anything. Imagine trying that as a man - it'd be along time before you got out of prison.