The world of Rune-scape has a Dark-age/Medieval theme, there are a great deal of skills you can do, if you don't like "mindless slaughtering" or just want diversity in your game play. It's true that the members have a much larger area, but it you play F2P (free to play) until you complete all the free quests you won't be as bothered by it.
Rune-scape makes use of magic, prayer, demons and also the mythology of that world is Polytheistic. The extent of its religion is the different types of god armor you came get, along with some effects on magical spells. Once your child is of the age of reason, usually  seven, they should be able to tell the difference between your family’s theology and Rune-scape's mythology.
The Violence in Rune-scape is diverse but virtually no gore. There can be Violence against humans, dwarves, elves, animals, monsters, undead, demonic, etc... You can have pvp(player verse player) in addition to the pvm(player verse monster). Some creatures may or may not be frightening to younger people. Understanding the concept of real and fiction as about the age of reason as well.
There is drinking, but no drugs. If you do drink your character will have the effect of a strength potion (about), but at the cost of your attack. There is a quest that requires some smuggling. The language can be filtered, however some people try to be clever and use leet speak. It's not really a problem; you can ignore them or even report them. Do to the ignorance of others I would say about age (13) thirteen would be a better age to start.
There have been no sexual undertones that I can think of.
Rune-scape, as a safety feature, will auto log-out after about five minutes of inactivity. Jagex does a fair job on security; ultimately, if you don't follow the rules, you only have yourself to blame, for any loss to your character.
There are advertisements, which Rune-scape allows in order for it to stay free. ONLY use your pass on official Jagex sites. If you take the right steps you won’t have a problem.
Unchecked this game, as with any (if you like it), came start to be addicting. For how long depends on your interest and self-control.
You can play it in your browser and the graphics are kind of like claymation meets cartoon.