I have used Wordle as a quick and easy way to generate word clouds.
Imagechef is a nice alternative because it allows you to choose shapes for the word clouds.
Both of these are great for use in school, because they don't require an account to use their service, and they are very easy to use. They both work well for student use.
One weakness of these sites is that the word clouds cannot be viewed on a tablet.
I recently discovered Tagul, and while it is much more complex to use, it offers a lot more options as well.
Tagul requires an account, and has many more features (individual words in the word cloud are "tags", meaning they link to an external website). It is more of a tool for teachers, rather than elementary students. What I like about the Tagul word clouds is that they are viewable on a tablet, and the links work on the tablet as well. Unfortunately the word clouds can not be created on a tablet.