How to protect & preserve water color paintings
There are a couple of things that will definitely work to help preserve displayed paintings:


Paint quality. If you want the paint to remain vibrant over time, then you get what you pay for. Artist/professional grade paints are substantially more lightfast than student or children's paints. So, if you want it to stick around better, get the good paint.
Ultraviolet light protection. If you're going to display your work, display under UV filtering glass. This helps to preserve the colors from damaging light and, as an added bonus, protects from the touch of errant fingers. :)


You can also get UV archival fixative sprays that are suitable for watercolor work. It's not terribly common to do that, but becoming more prevalent amongst those that don't want glass in front of their painting. My general take is that if the paper gets dusty over time, dulling the color, it's harder to clean than glass... Mind you, for storage long term, it makes a lot of sense to consider.