there could be another reason for the increase in alumni contribution which is that as the child's age increase so does the alumnus' too and so does his/her income. Which can directly result in higher disposable incomes which in turn would lead to higher contributions to any charity including donations to college.
My personal thinking in this matter is that I would not donate to my college if I think that could have an impact on my kid's chances of getting an admission. It would be unfair on my part to think so and it would be even more unfair on the part of the college to consider the amount of my donations as helping the cause of my kid. I would like my college to take the best students no matter what their pedigree is and no matter what their parent's contributed to the college.