In statistical analysis you can never have too much information, at least in the modern age. I work with a lot of data and the more you can regiment data the more accurate it is. Usually, top level classes can be deduced from lower level classes.
Take for example address data. If I ask merely your state of residence I have no way of deducing your city or zip. If, on the other hand, I ask for your city and zip I can deduce the state. This is how polymorphic data works. You want the lowest level class. Everyone living in Parma 44134 lives in the state of Ohio while everyone living in Ohio doesn't live in Parma 44134.
I'd imagine they ask to have more accurate data, nothing more. In this case all adopted children are children while all children are not adopted. I have these battles all the time at work when data isn't collected at a low enough level to generate the reports they will inevitably as for later.