I don't think young babies "know" fear because I don't think young babies are developmentally capable of "knowing". But young babies can certainly experience the physiological arousal of fear. And because their brain (neural pathways/personality/sense of self) is developing so rapidly in that period of life the physiological state of fear will become a part of their being.
Which gives a totally different experience to PTSD (where a person's personality has changed from the pre-trauma "normal" state) whereas in complex PTSD there is no separate pre-trauma "self" because the neural pathways/personality/sense of self has developed to accommodate ongoing trauma.