I have a niece who's turning 6 in June, but reads at a 2nd grade level, and I got her "Marvel Action: Spider-Man TPB #1," but went through it with another relative, and she felt it might be too scary and advanced for her. Anyone have any other recommendations? She's really into female superheroes, and liked Spider-Woman in the "Spider-Man: Into The Spider-Verse" film. She also likes the "Super Hero Girls" cartoon (which isn't Marvel, of course).
You could try Moon Girl & Devil Dinosaur. The main character is 9, but it's still kind of typical superhero action stuff.
I'm not sure if you'll find any actual comics for her, if the Marvel Action didn't meet your ideals. However, there are children's books (like Golden Books and stuff) that feature Marvel characters.