Presumably, you'd get the pizza from a pizza place. If you go for a national chain, you wouldn't even be restricted to testing kids all in the same location.
I would imagine a kid would think they would be full after the first slice and not care about the second.
It would be better for the test to have the child rank edible items they like. Then a few weeks later run the test.
But as a new father, I've learned that eating training will vary widely between families. Some families do not provide sweets as treats at all. Some will always provide it after every meal so the kids do not feel it is a reward or scarce resource. You can read up on the Satter method for details.
