Is the kid okay at math? Only need one basic equation here.
Imagine a guy on a train throwing a ball straight in the air and catching it again. To someone on the train it looks like it’s just going up and down. But to someone standing by the tracks, it looks like it’s going in a big arc, because it also has the horizontal velocity of the train.
Now imagine the throw is at near light-speed. The person by the tracks would then think the ball is going over the speed of light. But that’s impossible.
Consider v = d/t. If we accept that v is fixed, is the speed of light, and we also accept that d is larger for the guy outside the train than it is for the guy inside the train because of the added horizontal distance, then the only way to make the equation hold is for t to be larger for the guy outside the train than it is for the guy inside. Time dilation occurs.