You can just request a child seat at the location when you pick up. Requesting it beforehand doesn't really do anything in terms of the location setting one aside for you or anything like that. It's more just a notation on the reservation. They don't often run out of them, so there's not much risk of that, and even then, they could always pull one from sister companies Enterprise or Alamo in a pinch.
There is no extra charge for a one-way rental. You just pay the daily rental fee+tax for it.
Hope this helps!