You may be able to remember that you need eggs, bath tissue and onions without a grocery list, but getting into the habit of creating a running list is a good idea. It saves wasted effort. It can also become the first step in an ongoing planning strategy your whole family can use to make notations of the things they need. Don't stop with a simple list, either. Teach your child to relate those items to other things. If he wants to buy prepared chocolate chip cookies, does he want some milk to go along with them? If so, how much milk? To discover the answer, he'll have to take an inventory of what's in the fridge before he heads out shopping. Another lesson learned.
If you're both making the cookies instead of buying them, this is a wonderful time to take an analytical look at a few recipes to understand how totaling quantities shows him how much of a particular item, like flour or butter, he may need. If you've ever been up to your elbows in holiday prep only to discover you didn't buy enough cream, a refresher course in aggregating quantities may be overdue for you, too.