Do you have any plants from which they can harvest seeds? We have had lower elementary students harvesting marigold and sunflower seeds and they love it! Cosmos would be another good choice. They could make and decorate seed packets (several online sites have templates you can print) and store their seeds for planting next spring. Many of our parent chaperones were surprised that you could grow plants from seeds you harvested instead of buying commercial seed packets. You might even be able to pull some plants from an off site garden and harvest from them if you don't have marigold (or something else) available. A few sunflower seed heads would be give you lots of seeds and kids love to pick them out. You would have to limit each to a reasonable amount so some would be left for others to harvest :)

If you want to go with pine seeds you may need to quick collect some cones and get them into a dry place so they will open and release their seeds. I have never seen them for sale in my area and would have to order them which would take awhile. Might want to check Wintersowing as I think the pines might do best if they are kept outside with natural conditions to help germination. I'm not sure if the long wait for germination would work well with children. If you are having drinks at the festival, perhaps you could "recycle" the cups as your pots. Just add some drain holes on the sides near the bottom edge.