I thnk it's obvious that the child seat is not the optimal place to be in a rear-end crash -- it is an a "crumple zone" of sorts -- but two mitigating things: the rear of the station wagon is not as sacrificial a crumple zone as a sedan's, as those "D" pillars are quite robust, far far more than the more flexible sheet metal of the sedan's trunk. And also (assuming you're not being rear-ended by a tall SUV), the lower rear of both sedans and wagons have a clever alignment of the lower frame* and a heavy piece of metal extending, via the rear brake calipers, to the heavy rear axle -- this absorbs and redirects energy of the impact into the massive axle, protecting the rear compartment's structural integrity.
[ * yes, I know the car doesn't really have a frame, but it has what would be called "stringers" if it was a boat, used to provide rigidity or, in the case of a car, dissipation of crash energy.]