I had thought of initially getting written permission from parents and approval of the photos by parents/guardians before use of any photos, if indeed I will entertain the subject.
You will definitely need a model release from the parents/guardians before publishing any pictures on the web.
Check with the school though as most get releases signed at the start of the school year (we sign one for our kids so that the school can post pictures up to it's web site), it just depends on what the release says but will most likely will cover them for web sites.
[edit] Tony beat me to it, but I was under the impression that taking pictures of kids is not illegal, but you weren't allowed to publish the pictures without permission?