You want ten of my chickens. We agree that, in exchange, you will make me a website for my chicken farm.
Then, you want ten more chickens. But I don't need any more websites. However, you have a friend who knits llama wool scarves, and needs a website. You make your friend a website; your friend gives you five llama wool scarves; you give me the five llama wool scarves and I give you ten more chickens.
When you want yet ten more chickens, you introduce me to your friend who grows corn. I wouldn't mind some corn, but the harvest hasn't come in yet, and I'd need to wait a few months. You want the chickens now. So you build a website for your corn farmer friend, who gives you an "I owe you" certificate for the corn. You give me the certificate, and I give you ten chickens. A few months later, I present the certificate to your corn farmer friend, who gives me the corn.
Eventually this becomes a pain to keep track of, and it becomes increasingly difficult to arrange personal connections with people who need chickens or llama wool scarves or corn or websites.
Money lets us exchange goods and services in the abstract. I give you ten chickens, and you give me money, which everyone else will also accept in exchange for goods and services. I can go across town and find somebody you don't know at all and buy a camera from them with the money you gave me for the chickens.
It is this memory that allows me to buy a camera from someone who doesn't know me. She knows that I earned the money somehow by giving something of value away to another member of society.
Would this be harder or easier to explain to a 3rd grader, and do you think this explanation has merits?
Is that what you're getting at? While perhaps a fine extension of the train of thought, I personally find that hard to think about, I suppose because, in practice, dealing in terms of memory money would be harder than exchanging documents, and the advantages of using money rather than direct bartering and trading become less clear.