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.
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?
Also, it obviously matters what it is you're saying. Most people just told jokes about the communist regime and talked about how shitty life was. Nobody cared about that as long as they kept it private.
And why would they (the KGB) have cared (about jokes)? The venting of steam, instead of letting it build up and affect change, would have to be invented if people didn't do it voluntarily. That kind of self-deprecating humour is all over our media these days, too, we love to bring up important subjects to channel them into harmless stuff, just because we can't pretend it's not there, but also not allow ourselves to actually see it and be serious in its presence.
But some assume that the absence of free speech means that everyone is either brainwashed or so terrified they are afraid to speak their minds at home. I just had to point out this is not how it was in the late USSR.
The point is also that there is a not that small category of people who are ready to believe really everything for as long as they hear that from a smart looking guy in a suite, and it is "dumb and simple enough to believe." This the very thing going on now in Easter Europe with ultraright maxim "refugees steal your welfare" and "Soros conspiracy"
Now, this is being weaponized big time, against the US this time. It took Kremlin probably just few hundred man hours to make one of such guys to attempt assassination of Hillary Clinton.
Now imagine how things will turn if they will double down on that.
In America, when children grow up, parents could have that talk with them about how George Washington really was a great guy and how much they appreciate if their children wouldn't cut down the cherry tree in the garden but also about how this story about George Washington is just that - a story.
I wasn't there at the time, so this is just my take on it: I imagine in the USSR it could have been dangerous to talk to children like that and one way out was to exaggerate the stories until the child notices what's going on. Now daddy Sergei Kurekhin was doing that to the whole Soviet Union - and that's why they laughed about it.
[1] I call it a joke and not a hoax, because I think hoax kind of misses the point. I wasn't there at the time but this is my opinion: I don't think anyone believed it, I think people didn't say so partly because they didn't want to destroy the joke and partly because this was still the USSR.