I'd say that it's pretty good and child friendly.
This game seems very good. It may be about assassins but killing can be avoided. In 'Career' mode, you have missions: eg. Stop the thief. In this there are different ways to proceed. 1. You could kill him. 2. You could use a smoke bomb. In career you are going to have to take out people sooner or later though. In Explore mode there's a whole different story. You don't have missions, just side missions (eg. Find the box and break it) it's more just exploring. You can still kill people, but there is no use in doing it. If you child chooses to kill people in explore mode it means he has a problem not the game. Overall it depends on what you've seen, and if you trust your child won't go Rambo and just kill. Overall it's pretty good, the controls are buggy but I would recommend for 11-12+. Also it does have a bit of blood, but that is only seen 1/2 of the times.