I pushed on WPI last year to get some certainty about future costs, and they were not very forthcoming. They told me “we have already taken into account when you will have kids in college” for the calculations. However, they also said, “Your financial aid will not ever increase while your child is enrolled. It is possible it will decrease if your circumstances change.”
Personally I didn’t find it very encouraging! OTOH, I have not seen anyone on the parent FB group complain about costs going up after another child graduated, so I don’t think it is something to at happens a lot or drastically.