According to the letter I have from the FAA, dated 05/02, the reg is that *ANY* CRS must be placed where it will not impede egress in an emergency. (The FAA's term, CRS, is for child restraint system.) This translates into a window seat on most aircraft, or in the exact center seat of the center section of a widebody. The seat may not be placed in an exit row, of course, but also not in the row in front of the exit row, because the backs of that row of seats are meant to fold forward to widen the exit row so that people can climb out that door. (If there is a carseat strapped into the seat, you can't push the seatback forward. An FA demonstrated it for me once.)
I have found that reservation agents and gate agents are often not sure about the rules on where the seat can go, and different airlines will enforce it differently. However, most FA's *will* make you move a seat if you put it anywhere other than a window or the center seat on a widebody. If the person you speak to on the phone says there is no such rule, it is best to ask them to double-check it while you wait.
PS: I should have mentioned the exception to the window rule.
According to my FAA letter, if there are 2 children in safety seats but only one adult travelling with them, then the first seat should be placed at the window, and the second right next to it, then the adult next to that seat. They did not tell me what the pattern would be if there were more than 2 babies in carseats with only one adult.
Also, another rule about multiple children: no more than one lap child is allowed in a single aisle-to-aisle row section. The reason for this is that each row section only has one extra oxygen mask, (that is, 2 seats=3 masks, etc.) So, if you and your lap child sat down, and then someone else with a lap child sat down next to you, the FA would have to make one of you move to a different row.
