For pre-teens there is a wealth of information on butterflies available online. A quick 'Google' for "butterfly facts kids" should get you off to a good start.
However, here are some more thorough sources:
kidskonnect.com starts with simple facts, but it also provides a downloadable resource pack. The pack includes a study guide, activities and a quiz.
kidsbutterfly.org is dedicated to butterfly information. It also includes links to teaching and learning resources (in a few languages).
Outernet Project aims to engage young people in learning about butterflies. They provide basic materials for butterfly catching and identification if you join the Lepidopterists' Society.
biokids@umich have a great website. The page is designed 'FAQ' style with links for further learning. The website also features a great gallery. The information here is initially very easy-to-read. However, very detailed and technical information is also available.