You should be pretty OK with Thor. There is some indication that there's some chemistry between the lead and the lady doctor, and a possible cross race kidnapping and hostage taking, but for the most part, it's all light...the violence is CGI and the heroes ultimately win.

The acting is good, they take themselves seriously, and the dilema the hero is placed it is not emotionally overwrought.

As a summer action film, it's good. It's not exactly faithful to the comic book version, but close enough. The changes they made work well.

I think I'd take the 4th grader with me, rather than turning him loose.

Want another good comic book movie that's good for adults or teens? X-Men First Class is a surprisingly good action film that paints the picture of how the X-men got going and how they pulled the Cuban Missle Crisis back from the brink.
There's no overt sex, though there are occassional nude female breasts and at least a passing adult tabletop dancing club and one person showing up in another's bed...but no action occurs... until the big fight scenes. It's very satisfying with the right hint of tragedy, pain, frustration and vengence. Oh, and there's at least two on camera killings, so be prepared or half your 4th grader prepared for Nazi's vengence and some adult themes.