Even the youngest children can create their own Seder plate that can actually be used at a Passover Seder.
Just show them the items (or photos) of what goes on the Seder plate, and let them be creative, drawing on a white paper plate with crayons or markers.
If you have indulgent guests, (grandparents are the best) you can have the kids make a "personal Seder plate" for each person at the table, in addition to the real one which sits at the center of the table. You can put some of the easy items on these personal plates at the actual Seder, like parsley or a slice of boiled egg (stay away from putting salt water on the paper plates of course, it will wreak havoc with the lovely marker-art and tablecloth)
You can even cover the paper creations with clear contact paper for a laminated look and protection for the artwork.
Older children can use white plastic plates and use permanent markers for a Passover keepsake.