One of the best movies of recent years. It's a lot of fun and manages to keep a balance between cool action scenes and great character moments. Lots of humour too.
The violence is frequent but pretty mild (not much blood/gore) but probably too much for young children, though of course it depends on the child. The only part that was a bit over the top was where Loki stabs some dude in the eye. It was pretty disturbing, but it's brief and not graphic at all (you see Loki grab the guy and hold the device up, then it cuts away and you hear him scream.) Another potentially disturbing moment: In Black Widow's opening scene she is tied to a chair with the bad guys about to torture her (by pulling out her teeth with pliers.) She escapes before anything can happen, but the threat could upset sensitive children.
There's no sexuality beyond a brief kiss and some slight innuendo between Pepper and Tony. Black Widow's uniform is skin tight, but everyone else's are pretty tight too, so it's not that big of a deal. Oh and Bruce ends up naked, but it's not a sexual context and obviously everything below the waist is covered up.
There's not too much language: 1 a**, 1 son of a b*tch, 1 b*stard and a couple of h*lls and d*mns. At one point Loki uses the word 'quim' which is an antiquated form of the c-word but most children probably wouldn't get the meaning. As far as profanity goes it's mild, but it could still be considered inappropriate due to its usage as a misogynistic slur.
There are a couple of scenes of drinking and a mild drug reference but they're so brief and in the background it's barely noticeable.
Positive messages about teamwork and sacrifice abound. All the Avengers are good role models, particularly Cap.