Besides everyday "practical" lessons such as household safety and food handling, young children that spend time in the kitchen begin to develop a whole set of math and science skills before they even reach elementary school. In helping a parent cook, children can learn about measurement, fractions, weight and volume to name a few skills. They also better understand basic scientific method, principles such as cause and effect, they dabble in chemistry, and learn some basic laws of physics such as how the same substance can take the form of liquid, solid or gas. Just think about baking a cake, that process will likely touch on every item listed above.
Speaking of cakes, if you want to gear a cooking project towards something that is more of a fun lesson rather than making dinner, one of my favorite activities in the kitchen is to make a "Rainbow Cake." There are a variety of recipes out there and different levels of difficulty. Just search "rainbow cake" on your web browser, and pick the best recipe to suit your needs.