I should probably know the answer to this, and maybe I do and pregnancy brain is blocking my memory, but how does a child's weight and/or head size factor in to their safety rf vs ff -if at all? I've seen several people mention their light weight kiddos or that they have a larger melon so it got me to thinking about all of this. Especially since I am about to have to flip 3.5 year old ds ff and reeeally wanted to rf until at least 4.