To date, since starting out with comics, through my time on ZIM and since, I can’t recall any kind of communication like what you’re describing. As parents who know about my work go, most, thankfully, have better things to do and far greater things to worry about than to write angry letters at me.
The only “parent letters” I’ve ever received on the subject of their children have been overwhelmingly positive, whether it’s about how some of my work inspired their kid to start making comics or made them want to get into animation, or some such thing. I’ve gotten mail from teachers describing students who opened up a bit because they were into my comics and, through comics, they were able to deal with some issue or another.
Now and then I hear about parents who were themselves fans of my comics from back when I was first doing them, and, despite being fans of my comics, managed to get someone to sexilize with them and they procreated! These parents tell me about how they sit with their kids to watch ZIM, they’ll tell me what parts the kids laugh at and it’s pretty awesome.
Was any of this my intent when I was making all that stuff? Not at all. Hell, I was too busy giggling while writing something or drawing, the exact same way I do now when I’m working on something, to even think of a surreal future where people are digging my work with their kids, or, in some cases, waiting until the kid is old enough to show them the more intense stuff. Either way, it’s cool to hear about.