First off: If you don't think your child is mature enough to virtually beat a cop to death with a crowbar, this most likely isn't the game for them. (Or, to be more accurate, for you.) I know that I probably just scared you away from this game, but I'll continue anyway.
This is one of my favorite games of all time, if not my absolute favorite. The game is awesome. (If you're mature enough to play it.) It's just as simple as that.
Now as for the violence, I believe another review already pointed out that their is an option to turn the blood off during gameplay. (You should know that this is only available in the PC version, not the consoles.) Even when the blood is on, it's not really that bad compared to other FPS's. When an enemy is shot/hit, a very small amount of blood actually come from the actual character. Most of the blood you will see will be on walls and floors. It sounds worse than it actually is. If you're wondering, the blood does look realistic. It's a realistic game. That's the whole point. Weapons you use are: A crowbar, a pistol, Magnum revolver (Dirty Harry gun), crossbow, grenades, sub-machine guns, shotgun, rocket launcher. The enemies use sniper rifles, as well as some of the above. Citizens are beaten with stun batons by cops. Enemies are always either trans-human cops/soldiers bearing gas masks and Nazi-like uniforms, zombies, or monster-like alien creatures, including "headcrabs" that latch onto people's heads, turning them into zombies. (Which you then have to kill.) Zombies and blood are usually working together in this game, as they (the zombies) can be found lying down in a puddle of it (a result of the headcrab attack) before getting up and trying to claw at your face. One of the levels is entirely zombie-oriented, as is by far the bloodiest. That level is not for the faint of heart. If your child is easily scared, make sure to tell them not to go to Ravenholm.
Moving on, I'll explain each of the concerns/highlights that I labeled at the top of this review:
Bad messages: Might teach kids that all problems have a simple answer: Hit it with a crowbar.
Good role models: The character you play as is Dr. Gordon Freeman. He is well known for being resourceful in times of extreme pessimism, as well as being a good leader. (Earning nicknames like "the One Free Man") He's also a mute and has a PHD from MIT. This makes him unique from the classic soldier kind of video game hero, and because of this he's a bit of an icon in the video game community. After all, just look at my profile picture.
Educational/Safety Issues: Though these wouldn't normally go together, I'd just like to point out that I know a lot about weapons, and I didn't before playing this game. However, this probably has less to do with the game itself than the fact that this was the first FPS i ever played. (And the first 'mature' game.) If their is any risk of your child (presumably younger) screwing around with guns, (i.e. you keep a handgun in your basement 'just in case') they shouldn't be playing video games in the first place. More importantly, if you have a kid, you shouldn't have a gun at all. But I won't get into that...
As a final note, I would like to discuss the general atmosphere of the game. From the moment you arrive in City 17, you have this feeling that something has changed; something is wrong.
The game takes place in a near-future earth that has been forcefully taken over by an inter-galactic empire known as the Combine. To citizens they are represented by the metro-cops that walk the streets, terrorizing anyone and everyone. Humanity has been completely enslaved, except for Dr. Breen, the (human) figure-head leader of Combine earth, and the One Free Man. Dr. Breen records "Breencasts" which are displayed throughout the city one huge monitors, spewing propaganda in an attempt to shape the minds of the people below. Due to Combine technology, humans can no longer reproduce, ensuring that the current generation will also be the last.
There ain't no rainbows in this game. Just the mood might be too much for some younger players.
Final assessment: Iffy for 12+, Probably fine for anyone 14+, -- Definitely OK for anyone older than that.