Technically, the exit row *and* the rows directly before and behind it are normally forbidden rows for carseat installation on US carriers. Some airlines may not enforce the rule about the row behind the exit row, and to be honest, I've never been able to find an explanation about why carseats should not be in that row. However, the FAA definitely requires that carseats not be directly in front of an exit row, b/c it could interfere with being able to open the door. The row of seats in front of the exit row has seatbacks which are set to be able to collapse forward in an emergency, so as to widen the crawlspace out of the exit, and to provide better maneuvering room to open the door. If there is a carseat strapped in there, you can't collapse it forward.
Carseats must be positioned where they do not impede egress from the plane (translate that as "where no adult would have to crawl over it to get out of the row.") In practical terms this means that on a widebody, the seat can go in the middle seat of a row in the middle section, otherwise, and on all single-aisle aircraft, the seat has to go in a window position, *UNLESS* you have more than one seat and only one adult, in which case the seats should be installed as window and center, with the accompanying adult on the aisle. (I've never seen anyone attempt to put more than 2 carseats in one row, but I suppose you could do it on a widebody if you had an adult at either side. I don't think more than 2 with only one adult is allowed.)
Also, though carriers won't tell you this, a word of warning about using a carseat in a row directly in front of a fixed bulkhead; try hard to avoid it! Very often, the only way to release a strapped-in toddler carseat when the buckle is behind it, is to reach in between the seat cushions from the back, which is pretty impossible if there is a wall behind it. TWA assigned me that spot once, and I had to wait for a mechanic to come & disassemble the seat so that I could get the carseat out. Beware of getting assigned that last non-reclining row.
BTW, There is also a rule about how many lap-babies may be seated in an otherwise completely occupied row. The limit is one lap baby per row section. The reason for this rule is the oxygen masks, b/c there is only one extra mask per row section. If you had two adults and two lap babies (say, a couple w/ infant twins) the parents could not be seated in the same row unless they split across an aisle.
