[Umpteenth rant that's crossed my dash about "impressionable children being sold the images and logos of fictional extremist hate movements" re Hydra and Death Eater merchandising]

I'm going to break this down for ease of argument.

1. Fandom is for everyone. Children are welcome, children belong here, but trying to rid fandom of everything (and everyone?) that you'd rather children didn't see is unacceptable. I don't care how self-righteous you feel about your cause. That's major douchebag behavior.

2. Children run into problematic stuff. As long as problematic stuff exists in the world, children will notice it, because they are awesome like that. Trying to keep them away from it just makes it seem cooler and more attractive. I'd hope that's not what you're going for. 

3. Some fans, children included, like villains for all sorts of reasons. They're not obligated to explain it to you, and they have as much of a right to like what they like (without being concern-trolled and accused of immoral behavior) as any normative hero-fan. 

4. The explanation that people throw around "ohnoes, they must like death eaters because they're racists" is a chimera. Racism, sexism, and other institutionalized abuses of power exist in fandom, but asserting that they're interest coded is beyond naive. 

5. Oppressive people often get the benefit of the doubt because their favorite characters are socially-approved. Meanwhile, people who are bothering no one are chronically suspected of being horrible because the group asserts that who they like is a more reliable indicator of their character than how they actually treat other people

TL;DR, Challenging villain-fans' right to exist in public and be visible is overt bigotry. Instead of hand-wringing over the perceived image problem they pose or the fact that companies are willing to take their money, fandom needs to address the broken dynamics of its own geek hierarchy.