Harry didn't see his mother die, it says so on the JKRowling website. He was in the same room, but in his cot (or whatever you call it). Which makes the scene in the PS movie a mistake...
Also on the JKRowling website, it says that Harry DID see Ced die, but he was to preoccupied with that fact that he didn't notice the Thestrals.
I think thats just an excuse though, she probably just hadn't thought of the Thestrals yet, like iluvsirius and others have already said.
I love your name, iluvsirius!
😄