Having been a very frequent user of the site for almost two years now, I think deviantART is a great site, and a valuable resource. The site features a vast range of artwork from all skill levels- amateur to professional. I even know of a few animators for Disney/Pixar who post their sketches and personal work on the site.
DeviantART has value to just about everyone in one way or another. Kids can look at fanart for their favorite cartoons, or get inspiration from brilliant illustrators specializing in cute fluffy animals. Students can use deviantART to upload work they've done for school projects, or on their own time, and get valuable feedback and critique on it, as well as learning from many tutorials and guides for improving their art or writing, and for accessing a wide variety of stock images to use as references for their own work. The overall atmosphere is constructive, supportive and fun, and the community is quirky and eclectic, promoting diversity and personal growth.
This said, deviantART is still not a site for everyone, especially young kids. Though users are generally responsible with what they post, and are careful to place 'mature content' tags on art banning younger users from viewing/reading it, there's no denying that deviantART shows the darker underbelly of art at times. Some art is highly sexual or violent, featuring nude figures (usually women), excessive gore and alcoholism/drug abuse. Heated debates have occurred in the forums discussing whether the admins allow porn on the site, masquerading as art, and admins are sometimes accused of playing favorites among artists & genres. And, as with every unmonitored forum site, there are some badmouthed jerks posting hateful and inappropriate comments, founding crude, tasteless groups and being generally unpleasant.
All in all, deviantART is a fun, constructive, helpful site with a few very rotten seeds mixed in the bunch. I highly recommend it, but only for older kids or people who can overlook shock-factor artwork.