How do I waterproof DD's twin bed? (x-post w/AroundTheHouse)

[MommyAllison]

We've got the mattress and the fitted sheet, but what is the easiest, best and cheapest way to waterproof it? Is there a great waterproof mattress pad for twin beds? I would think so, with all of the bed wetting kids out there. I'm hoping to get something at Target this afternoon. Thanks!

Allison
Mama to DD 11/05