What about these verses?
Happy birthday huh! Happy birthday huh!
Women crying in despair, children dying everywhere
Happy birthday huh! Happy birthday huh!
Your servants steal your wife’s untrue, your children plot to murder you
Happy birthday huh! Happy birthday huh!
On the ground is blood and eggs, Easter Bunny broke his legs
Happy birthday huh! Happy birthday huh!
Burn the castle storm the keep, kill the women but SAVE THE SHEEP!
When I was growing up in St Louis in the early-to-mid 70s one of the local TV stations had an afternoon kids’ show that was hosted by a clown and featured a kid with a birthday and all of his friends in the studio. At some point in the show they’d all sing “Happy Birthday” to the birthday kid and the clown would add “cha cha cha” at the end. But that was forty years ago and I don’t ever recall actually hearing people do that in real life.
I have a nephew that ALWAYS does that.
I have gone to my little cousin’s birthday parties (she is 5 now), and have never heard that added to the song.
A restaurant chain called “Happy Joe’s” was a popular kids birthday party place in Wisconsin when I was growing up. The wait staff would sing Happy Birthday in the “cha cha cha” manner but instead of singing cha cha cha, one of the wait staff used a circular honker horn (French horn sized) in place of the cha’s.
I see from The Google they are still around and still have the birthday horn. 
My kids (6 and 4) picked up cha-cha-cha somewhere unknown to us – it’s never been sung at any family birthday parties (and we have a lot of those) so they either got it at school or at friends’ parties. It’s stupid and I hope they outgrow it.
There was a – mercifully brief – time when “cha cha cha” was humorously added to any song. Fads come and go.
It’s like appending “shave & a haircut” or “and many more,” or “speech!” – same fad.
I’m trying to think of a cha cha cha recording example that’s playing in my head, but I can’t remember the song that preceeds it! It was a 1955-1962 rock tune.
The kids in my family add “cha cha cha” every time"Happy Birthday" is sung at a party. I don’t remember my siblings or cousins my age doing this. My brother would sometimes add “and many more, on channel 4, with Mickey Mouse, and Donald Duck, and channel 9, with Frankenstein…”
I’ve heard it at every birthday party for at least 12 years. Even 4 year old kids seem to know it. I just assumed it was an american thing.
Wow, I’ve never heard of adding cha-cha-cha to the birthday song. I have heard of the monkey version though.
My family does something far worse and annoying. After the birthday song they sing “Are you one? Are you two? Are you three? Are you four?” etc until the age of the birthday person. It’s especially horrible now that everyone except me is over 40. Does anyone else do this? I’ve never seen it outside my Dad’s side of the family.
You’d have to be quite a lightweight to get plastered at a rate of one beer every year!
Mule Skinner Blues, by the Fendermen concludes with “Cha Cha Cha,” but only once, in 1960.