This is one of those times when I get to put on my ASTM hat and answer from that perspective in terms of lightfastness. The Colored Pencil Society Of America (CPSA) worked with ASTM over the better part of a decade to establish a rigorous lightfastness testing standard, ASTM 6901, that subjected color pencils to the same conditions and total exposure levels of light and UV, in both outdoor and accelerated indoor tests, that artists paints have been tested under. This was originally done with the participation and support of a large number of manufacturers in the color pencil industry - Prismacolor®, Derwent®, Van Gogh®, Caran-Dache®, etc. This was truly a chance to put this medium on par with artist paints, which had long set the standard for lightfastness. While initially this work did lead to several companies adopting the standard, over time one after another has ceased to use it and currently most, if they actually do any lightfastness testing, will claim to use the Blue Wool Standard, which is certainly better than nothing but is no where near as rigorous or controlled as the ASTM tests. With Blue Wool, unless you know truly what the test set-up was, the nature of the light, and the strength and duration of the exposures, the results simply are too subjective and variable to be relied upon in my opinion. So that's the bad news. On a more positive note, Caran Dache's Luminance 6901 still holds to the standard and even prominently displays its conformance to ASTM 6901:
There certainly are other factors (feel, color range, hardness, etc.) for choosing a color pencil line, so this is in no way saying that these are the only pencils to use, but in terms of lightfastness - and speaking as a member of the ASTM Subcommittee for Artist Materials - I generally recommend choosing those materials that conform to an ASTM Lightfastness Testing Standard.
That said, I do believe that Prismacolor at one point had a line that conformed to ASTM D6901, so depending on when you bought your pencils, you might have ones that were tested under those protocols. You might call Prismacolor and ask, or give the CPSA a call as the folks there are very helpful and knowledgeable about lightfastness and even continue to conduct their own tests.
So that takes care of point one.
As for protecting them, I feel they really need to be mounted under museum-grade UV protective glass or acrylic sheeting. Something similar to Tru Vue® Optium Museum Acrylic, which filters out 99% of UV, would be our recommendation. That particular glass actually performed incredibly well in testing we did looking at the protection of watercolors, including ones that were less than lightfast, such as Alizarin Crimson and even the very fugitive fluorescents. Color change was held to a minimum in most cases, and even with the fluorescents after 1200 hrs. in a QUV machine, the changes were at most just over 4 Delta E. You can see a graph of the results here:
To get even close to that level of protection with a UV Varnish, we had to apply 4-6 sprayed coats, which really leaves you with something that looks laminated. As you will see, just placing it behind regular glass did very little, and just light applications of varnish were also not effective at protecting anything with less than ideal lightfastness. On top of all that, applying a fixative or varnish will - in my opinion - always entail risk that the look and nature of the artwork will be altered. Keep in mind, too, that colored pencils usually contain waxes and so might be sensitive to solvents, and even the color and appearance of the paper can be affected. Plus any direct application of a fixative or varnish would need to be considered permanent and nonremovable. So test, test ,test before using on anything of value. And if needing to store the pieces flat or rolled up, perhaps look at a good cover sheet, such as a silicone release paper, which can limit the danger of smudging and protect the piece from dust and dirt.