Transplant them. They probably won't all live, but you can say you tried your best to save them.
I would use some small object, something like the pointed end of a plastic plant stake or even the tip of a flathead screwdriver, to gently pry each seedling out of the soil. Push the end of whatever implement you use into the soil beside the seedling, and gently lift the plant using a prying motion.
Repot it in a small container, such as a little paper cup. Use a pencil to poke 3 or 4 holes in the bottom of the cup, then fill it with new soil. (Use a potting mix, not garden soil.) Moisten the soil thoroughly, and set the cup aside to drain. Poke a hole in the center of the soil with a pencil, gently place the roots of your seedling into the hole, and use the pencil to firm the soil around it. (I do this by actually poking the pencil into the soil near the seedling, and gently pushing it toward the plant.) Then, water again to remove any air pockets that might still be in the soil.
The alternative is to pick the strongest seedling and pull or cut off the others. I have a feeling your son wouldn't appreciate you doing that, though. If he's anything like my son was, he's very
proud of having grown so many seedlings.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams