Purgatory can be a topic of confusion for our young Catholics. If God forgives our sins, why do Catholics believe in Purgatory? An article at LifeTeen provides a nice summary.
The short answer is that our sins are like breaking a window. God forgives us for doing that, but the effect is still there. The window is still broken. Purgatory is where that effect is “fixed up”.
The article provides the scriptural basis for purgatory along with quotes from the Catechism of the Catholic Church and a nice assortment of quotes from the saints. This makes a nice resource for talking to young Catholics about Purgatory.