I absolutely LOVE your World Series explanation/comparison.
It makes it easier to explain to the grandkids.
.
Then, once you have them somewhat understanding, explain that if we only used the popular vote to decide presidential elections, the candidates would only need to care about the desires of the people in a small number of very populated cities in a handful of states. What the vast majority of the states wanted would be totally irrelevant.
That's when you go in for the kill by using their own rhetoric against them and telling them that, "It wouldn't be FAIR for only 3 or 4 states to dictate to the rest of the country, now would it?"