I bought this for my kids and also have played through 80% of the game myself. We've enjoyed the PC versions of Harry Potter. The big attraction with this Wii version is the ablity to actually cast spells using the remote as a wand. If your 9+ child enjoys Harry Potter, they'll love this game just for all that wand waving. Younger kids may have trouble with the coordination and the level of logic and thought that's required to complete this game.
There are very few battle sequences and you "win" when the other characters fall down. No blood, no contact. Not particularly "violent" to me.
There are mini-games and puzzles that kids will enjoy along the way. Because the game is non-linear, players can do as much or as little as they want. Some kids might enjoy just walking around.
Now my negative thoughts. As part of the background action, students are walking around all over Hogwarts and you can hear them talking. The Slytherins are relentless in their insults. Frankly, as a mom, I get really tired of hearing voices coming out of my tv saying over and over "can he look any more stupid?" There are *lots* of these, so you might want to talk with your kids about what is and is not acceptable. The put-downs really wear me out.
Another issue I have is actually a bit ironic. The game ony allows for 3 saved games at a time. So if you have more than 3 kids that want to play, one will have to wait (maybe weeks, depending on how fast your other players complete the game). I can't understand why EA would do this, except I was tempted to get another copy for our family.