I'm not familiar with the size of Pizza Hut's personal pizzas, but the pizza I had at Pizzafari was about the size of one of those DiGorno individual frozen pizzas. I'm guessing maybe 8 to 10 inches diameter.
It fits almost perfectly in what I think is an 8in paper plate. The kids pizza is significantly smaller maybe 1/4 the size of the adult pizza plus its thinner. Each quarter can hold only pepperoni basically. If you are paying OOP you would definitely be better off having the kids share an adult pizza.