This is actually a really good question with a rather complicated answer. The root of it is how tire rubber interacts with the road surface; I'm not sure how old your grandson is, but if you remember back to high school physics, how you'd calculate the force of friction by multiplying the weight on a block by the coefficient of friction? And how you put twice as much weight on the block, and double the force due to friction? Tire rubber doesn't work like that. The coefficient of friction actually reduces with force; the tire starts out with a lot of grip immediately, but not as much more as you push it harder into the ground. You'll get more friction from a tire if you double the force (weight) on it, but not twice as much. So, simply put, a tire with a larger contact patch provides more grip than a tire with a smaller contact patch. Cars with a lot of power need a lot of rear tire to put that power to ground. Then you get into slip angles and how the tread deforms under stress and why wider is better than narrow for cornering, but that gets pretty deep down the rabbit hole.
So why aren't the front tires just as big as the rear? There's a balance between more tire for more grip, and too much wheel/tire making the car heavy and sluggish or unbalancing the car's handling. You see the most extreme example of this with drag racers, the fastest of which have dinky (almost useless) front wheels. For autocross and road racing, the front tires are usually pretty large to ensure the car has the right balance in cornering and often pretty close (if not identical) front to rear; a lot of front wheel drive cars even have larger wheels in the front and smaller in the rear.
Funny car: (note the low rear tire pressure and big contact patch vs the tiny front wheels)
And on the other end of the spectrum, a front wheel drive autocrosser: