OP needs the car seat (actually needs FAA approved car seat) for the aircraft if the infant is going in her own seat.
While it isn't the FA's job, it is the FA's job to facilitate the boarding process and if they don't help, just take your time and, trust me, a FA will help quickly enough.
If you want to be certain about all of this, comandeer a wheelchair pusher for a nice tip. They have SIDA badges which get them down the jetway.