Saw it last night. Great movie, but I'd hesitate to take a 7 year old because of the violence (some of it shown, most of it implied).
Honestly, I thought it was good enough that if you've got the time, check it out yourself and see if it's too violent for your wee-un. It probably won't bore you on a second viewing.
Best critical review of the movie that I've read: http://terribleminds.com/ramble/2014/08/03/an-academic-critique-of-the-film-guardians-of-the-galaxy/
I saw it and had a great time. I really liked how NOT cliched it was.
The violence - I wouldn't take a kid to see it. A bit too much. While you don't see someone get their head smashed by a giant hammer...it does happen off screen so you know it occurred.
I loved the movie and I am not a huge comic movie fan.