6 year old is a bit of a tough territory here, stuff like Rumiko Takahashi works (Ranma 1/2 and Inuyasha) almost seem like they’d be fine but they probably tackle slightly too mature subject matter and would be better in early teens age.

This is a bit of an old school recommendation, but you might enjoy checking out the original Astroboy, old Tatsunoko shows and stuff like that. The older anime tends to skew a little less “mature”. However, sensibilities of what is ok for kids in America vs Japan always makes this difficult. For example, Detective Conan is a great kids anime by Japanese standards, but showing murder victims and kidnappings might be a little much by American standards. So YMMV.