Why did the people prostrate themselves?
Rashi: On the three pieces of good news that they had just received: The redemption, coming to Eretz Yisrael and the children that they would have.
Tosfos ha'Shalem (7, citing R. Efrayim): They thanked Hashem that the Resha'im were buried during the darkness, so the Egyptians did not notice that also Yisrael were afflicted. This shows that if evil befalls a person, but his haters do not know, he should rejoice!
Rashi writes that one of the reasons they bowed was the good news that they will have children. They heard about an evil son!
Tosfos ha'Shalem (10): Moshe saw that they were pained about the evil children, and told them 'tell them "Zevach Pesach Hu... v'Es Bateinu Hitzil", and they will repent.'
Tosfos ha'Shalem (8) #1: They did not realize that he is an evil son.
Tosfos ha'Shalem (8) #2: One must thank for evil tidings just like for good.