Not always, but yes they most definitely do. AA seemingly assigns "next available seats" in order. So the system might assign you one seat. If the "next few seats" are already taken up by other passengers (who might have pre-selected/bought those seats) then your wife and kid will be assigned the next available seat i.e. not next to you at at all.

For my wife and me we don't bother pre-selecting seats, we only buy if we get split up while doing the web check-in.