Hey kiddos, be sure to get your parents permission before searching the web! They may want to learn something new with you!! :)
Noteflight is a composition website. This is a great website for 4th and 5th grade students to experiment with writing music. You have to create an account but the good news is that it's free!
Phil Tulga's website has fun and interactive activites that allow students to explore the fundamentals of music. We have visited this site in some of the music classes already, this year, and the students loved it!