Krylon K01309 Gallery Series Artist and Clear Coatings Aerosol, 11-Ounce, UV-Resistant Matte Spray Paint, 11 ounce, 11 Oz

I would probably go the route of an acrylic sealant spray between layers that you want to set. I have liked krylon matte sprays like this one in the past for jobs that needed a permanent layer of sealant. I have had good luck with cheaper options like this Rustoleum option. It helps to not use a gloss spray sealant for anything other than the last layer if you want a glossy finish. You want a little bit of grip for your pastels to adhere to and the matte spray can help provide that.

I wonder if a UV resistant clear coating would work to prevent degradation (also available in glossy, but I figure matte is better for kids equipment)