why the ministry didn't punish the Malfoys for following Voldemort?

Narcissa lied to Voldemort in the Deathly Hallows when she сказал(-а) Harry was dead...that kept them out of Azkaban after the Battle of Hogwarts.

I guess they were afraid of voldemort and everything was in chaos once the death eaters took over

'cause the Ministry is stupid

lucius malfoy donates a lot of money (that is mentioned in HP5)

Lucius was punished, he was in Azkaban. Narcissa was'nt really a death eater she just went with it because her sister, husband, and son were. And Draco I think he was forced to be a death eater and didnt want to be.

After the first fall of Voldemort?
By my calculations because Malfoy Ministry bribe! So Lucius as the ministries in charge! : P

If after the секунда fall of Voldemort, I know they have been indicted in Azkaban, but I'm not sure! : (
anyone can tell me?

I think it was because the Ministry realized that, in this case, they'd already suffered enough.