Okay this will be a very very generic explanation, because I don’t know a lot about plate tectonics, but it actually is something I’ve explained to four year olds before. Usually in the context of earthquakes, so that’s what I’m going with here
Hey [name]! You know how we had that big earthquake yesterday? You want to know something really cool? We don’t have any volcanoes near us (very important to lead with that if it’s applicable to prevent the kid from worrying about volcanoes/demanding to see a volcano), but the same thing that causes volcanoes causes earthquakes*! We live on a planet called Earth that’s shaped kind of like a ball, and in the middle of the ball is all the hot magma that comes out of volcanoes as lava (depending on the age of the kid, the magma/lava distinction is maybe unhelpful in the explanation). Magma is super super hot rocks, so hot they they melted and are a liquid like water! We live on what’s called the earth’s crust; that’s the outer bit of the planet that’s cooled down and turned solid (depending on the kid, pause here to talk about how different life would be if it wasn’t a solid; swimming to school, floating on the ground, etc). It’s kind of like the peel on an apple or an orange; the inside of the fruit is different than the outside.
The earth is different, though, because the crust of the earth is not one solid piece like the peel of an orange. The magma in the middle is moving around, and that means the crust moves (maybe pause here to get a bowl of water and like a leaf or something, and show how swirling the water makes the leaf move). Magma is a lot thicker than water, though, so it moves very very slowly. You know how [chocolate syrup/maple syrup/other liquid the kid knows] moves differently than water does? Magma is even thicker than that, and moves veeeerrrry veeerrrry slooooow (make a funny voice for that bit). That means the crust, the part we live on, moves too! The crust is very very big and split into a few different pieces so that it can move. Those pieces are called “plates” (pause here to talk about how silly it is that they are called plates and what the kid would name them, if they should be called bowls instead, etc if the kid is losing interest). We live very close to where two plates meet! Plates near each other can move in a few different ways (demonstrate with your hands). They can move apart, which makes those super deep underwater trenches where a bunch of cool weird animals live, or they can move together and both go up where they meet, which makes mountains. They can also move together and the edges of one goes under the edge of the other; that makes the under one start melting, which can cause volcanoes**! The one we live near is two plates sliding next to each other. They get stuck, and the magma under them makes them want to move, but they can’t because they are stuck! So the force of them wanting to move builds up and builds up until bang! They get unstuck and slide a little bit more! We feel that as an earthquake
*In my experience, every single little kid I’ve ever met thought volcanoes are really cool. None of them have ever been anywhere near a real volcano though, so like maybe don’t go with this as an opener if the kid has ever been near a real volcano, and especially not if they’ve lived through a volcanic eruption? I was born and raised in SoCal and am very very familiar with living in earthquake country, but not with volcanoes
**that...might not be the right explanation? Ideally, I’d google it first, but I’m answering these off the cuff the way I’d probably have to with little kids
