Same here but in the presence of a flight attendant I got up, looked straight at the kid and told the parent that their child had to stop kicking the back of my seat and behave.
I’ll do that next time with the flight attendant in our presence. In case the other party makes stupid accusations, it’s gonna be “he said, she said”. The last time it happened I had a 12 hour flight to Hong Kong a year ago, I was THIS close to strangling the little brat. I’m not even sure how they got seats on Business Class for crying out loud..