On a floured surface, roll out puff pastry sheets into 12- by 18-inch rectangles. Slide them onto a baking pan and place in the refrigerator. Let them settle about one hour, until firm.
While the dough is settling, make the tuna salad. Drain the cans of tuna and mix with the onions celery, mayonnaise, and salt and pepper. Set it aside.
Using a floured fish-shaped cookie cutter, help your kids cut out 16 pieces from the pastry sheets. (If you do not have a fish-shaped cutter, draw a simple fish on a piece of cardboard, cut it out, and use it as a template.)
Divide the filling evenly among eight of the fish pieces, leaving a 1/4-inch border all around. Place a cheese slice on top of the filling.
With a pastry brush or small, clean paintbrush, brush the 1/4-inch border of the pastry with the egg wash.
Place the top pieces on the filled bases and lightly press together the edges with a fork to seal in the filling.
Brush the tops of all the fish with the egg wash. With the tip of a knife, poke a small hole to make an eye. You can also lightly cut a fin shape, being careful not to press the knife tip all the way through the dough.
Bake for about 20 minutes until well colored and crisp.