1 box of gelatin mix, we use unflavored.

10 cups bird seed, we use Pennington birders blend. You prepare the gelatin as directed on the box (the one I get calls for 1 cup cold water and 1 cup hot water). Then mix in your bird seed. We usually do 1/2 of it at a time, so half the gelatin and 1/2 the bird seed. I spray cooking spray in the eggs so they come out easier and then pack with the mixture. Put in fridge to let set up (I put our in a glass container as some of the gelatin might come out the holes in the eggs).
10 cups makes 3-4 dozen normal size eggs. We do the bigger ones as well and it all stays together.

Edit: You can make this educational with counting for the younger kiddos. Or measurements or figuring 1/2 the recipe for older. We also hypothesis on type of birds and how long the egg sill last.