Yes, some jokes were okay, especially the toothless ones. Like, making a joke about being poorer or having worse quality of products than the West was relatively safer than making a joke about party members killing each other or hurting random people.
It also depended on who could hear you, and what kind of a job your parents had. The better job, the greater risk of losing your job for saying wrong stuff.
Also, "speak quite freely at home" is more than just jokes. I am pretty sure most people would not feel safe discussing The Gulag Archipelago at home.
I am sure there were kitchen conversations even under Stalin, but to far less extent -- most conversationists ended up in some unpleasant places.
Will you tell us next that Holocaust is just some Zionist propaganda?
First, you need to make sure your children understand the magnitude of danger from repeating what they heard at home. That requires some maturity and intelligence, but more importantly the idea that "there are important truths that you can get punished for saying in a wrong place" is itself one of those important truths that you can get punished for saying in a wrong place, so you either need to approach this very very carefully or take a risk and hope for the best.