instead of just keeping the paper yourself, you and your friend both get a copy of the paper. That way, you both have a record of all the toys that have been traded, and you can both see who has which toys at any time.
Now imagine that you and your friend want to add more friends to your toy-trading game. You can easily do that by giving each new friend a copy of the paper with all the toy names and who they belong to. This way, everyone knows what toys are available and who they belong to, and you can all trade toys safely and fairly.
That's basically what a blockchain is! It's a special kind of notebook that helps people keep track of things, like toys or even money. And just like how you and your friends had copies of the paper, a blockchain is a shared notebook that lots of people can access and see at any time.