Something easy for a kid to do would be making herb toast. Take some chopped herbs, (basil, thyme, garlic, scallions, chili peppers), mix them with a bit of olive oil, spread it on bread, then toast it under a grill. He can compare the taste of various different herbs with a control - toast with just olive oil. Since bread is pretty bland, he'll be able to taste the herbs. He could try dried vs. fresh herbs to see if there's a difference. Then he can get a bunch of people together and do a taste test. Extra points if he devises a rating system too.

I included a link below to the Herb Companion archives. They have recipes in every issue. Check them out for recipe ideas.

I personally would have a savory dish, a desert dish, and a beverage to illustrate the versatility of herbal flavorings. How about pasta with a nice pesto sauce, a nice iced herbal tea - hmmmmm, how about my recent favorite hibscus and lemon grass, followed up by some nice anise cookies?

FataMorgana

Here is a link that might be useful: Herb Companion Archives

Help him to make some bread, or scones, or even fresh pasta, keeping one lot plain, the other with herbs added. Savoury crackers are another way of doing it.

Another very simple idea is a jug of water, one just water, one with a sprig of mint, or lemon balm in it. Taste the difference! He could experiment with other herbs, too, like ordinary tea, basil, thyme, rosemary, whatever. Tell him he'll be making a cold-water infusion. That should blind him with science! He could have a row of jugs with a different herb in each, and little plastic medicine cups for taste-testing. He could, of course, make a real tea, using boiling water poured over the herb. He won't need to wait so long then for the flavours to infuse. Put a fresh sprig of the herb in the water for decorative purposes. Don't forget to label each jug so he can remember which herb is what.

Get some cream cheese. Make cheese balls. Keep some plain, to the others, in small batches, add an assortment of herbs, or just roll the balls in the herbs. Parsley, chervil, chives, sesame seeds, garlic (or garlic chives), paprika are some suggestions.

Make a small batch of spaghetti bolognese sauce, minus onion, basil etc. Just the meat and tomato base. Then take half of it and add in the herbs. Taste the difference!

I think eggs are the most neutral thing to really showcase the taste of herbs. If he is 13, he could certainly use an electric skillet. Scramble eggs plain. Then, scramble eggs with tarragon and thyme. A huge difference and a delicious one. You could then add any assortment of herbs. The cooking smell will get people to wander over.

If you use the egg example don't forget to include basil and marjoram as taste ingredients yummmy. Two of my favorites for eggs. I do like the scrambled egg idea because.
Fairly easy
easy to have a control group
bland enough to showcase flavors
smells throughout the room to create interest
lots of herb options to work with
Good luck and have fun he may just be the next great chef.
Herbs are how I got exited about cooking and 20 years later I still love to grow and cook with herbs.
Happy Growing and Cooking:) David