This site is good, for older kids. The profile slant can be definitely addicting, as certain achievements garner "badges". The all consuming hunt for badges is what drives most of the gamers on the site. Also, the chat feature can reveal some good or reasonable character, but it also can lead to kids jauntily sidestepping the profanity filter with the use of periods, spaces, misspellings and dashes.
A few games on the site are actually educational, like Globetrotter XL, a geography game that is actually fun, and Notecatcher, which teaches classical music with beats. But some games are violent, featuring zombie shooting, and some use of magic in the role-playing games. However, political games that jab at politicians, and sexually oriented games are never uploaded. The gamers on the site are encouraged to review games, and the uploader/developer will get points for how his game does, which prevents games that are not from being uploaded.
Overall? Just be careful about when they get on unless they are at least fourteen. This is the YouTube of flash games, and like YouTube, it can get iffy at times or places. Watch the forums, as well.