The difference between Spawn, Dredd, and Watchmen vs. Official Deadpool and Pennywise shirt is that DP is wrapped in a package that acts/looks a lot like many cartoon characters that are not as deranged as Wade Wilson. so even a kid that does know what to avoid, can be fooled by his goofy playful exterior. In the end, it comes down to whether you trust your child with absorbing this kind of material or not. How prepared are you to explain things that he does not understand. How well will he be able handle having his hero taken away from him. My suggestion, Provide his some more materials for more acceptable role models. Superman cape, Captain America shield or something like that. Try to pique his interest in those directions.
And I like to think I am a well adjusted member of society never been arrested, no anger issues, no major life issues one could attribute to a bad role model Official Deadpool and Pennywise shirt. I don’t even drink or smoke, despite Logan being the coolest thing in the world to me as a kid. My Dad had a way of keeping an eye on me, and if I emulated something I should not have, he let me know. He explained to me what was real life and what was not. He made sure I knew what words were okay to say and which one’s were not. I was watching R rated action flicks at a young age and reading lots of comics. My folks instilled in me a moral compass though that helped me know what i could and could not handle.