I doubt Simba knows about the Crocodiles. He arrives at the scene when the cubs are playing, so he couldn`t have witnessed the attack. Also I strongly doubt Kiara would tell her father about it, either as a cub or later as an adult.
As for Zira... I`m not sure. We see her lurking in the grass, watching the cubs.. But we learn in the next scene that Nuka was in charge of watching Kovu, meaning Zira was out on her own.. doing something. She was probably lurking around the border of the Pride Lands or something when she spotted her son. I don`t think she saw the crocodile attack, it seems to me as if she only witnesses the cubs at play.