Is this clail supported by data? It's really interesting but I've never seen studies that differentiated children survival rate accorsing to class (prospect was bleak in both cases). I guess that a nurse would be supervising the richer child and he would be less exposed to extenuous labor, but I figured it would show at a statistical level only after the industrial revolution.
Greg Clark has a lot of data A Farewell to Alms - basically lots of downward mobility until the Industrial Revolution.