I would create two templates for this game based on where the TN block goes.
If T is in the front car and N is in the middle car, we know that U also has to be in the front car (U
N), one of W/X will be in the middle car and one will be in the rear car (at least one boy per car), and S will be in either the middle or the rear car. Whichever car S is NOT in will have the empty seat (E
on my diagram stands for "empty").
If T is in the middle car and N is in the rear car, U will still have to be in the front car because it must be before N and since there must be two boys in the front car and at least one boy in each car, the middle and rear cars can only have one boy each. One of W/X will be in the front car and one will be in the rear car. That leaves S and the empty seat for the middle car.
Now question #4 asks: "Which one of the following is a complete and accurate list of the children each of whom CANNOT sit in a car that has exactly two children in it?" That's basically just asking us for a list of children who cannot be in the car with the empty seat. So let's use our templates to get rid of the children who can sit in the car with the empty seat. In template #1, N, S, P, W, and X could all sit in the car with the empty seat. In template #2, S and T are in the car with the empty seat. So the only children who cannot sit in the car with the empty seat are U and O, both of which are always in the front car which always has three children in it.
Hope this helps!