The big two culprits are direct sunlight and air.
You can buy UV glass and it will help, but the best way is to use archival quality paper. It is the acids in lower quality paper that reacts to the air and sunlight (Bernard, 2013).
The following links has great ideas for preserving your child's work:
http://www.myjrpicasso.com/index.php
Framing and Display Ideas:
http://blissfullydomestic.com/life-bliss/how-to-frame-an/1783/
http://projectnursery.com/2012/03/cool-tips-to-preserve-your-childs-artwork/