I am talking about the specific gesture of waving an index to show disapproval to a kid, often accompanied with the sound "tss". Is there a word or an idiom for that?
This gesture is called wagging your finger at someone. Cambridge defines it as:
If you wag your finger, you move it from side to side, often to tell someone not to do something.
The gesture of moving an index finger back and forth to express disapproval is usually called 'wagging a finger' or 'finger wagging'.
If you wag your finger, you shake it repeatedly and quickly from side to side, usually because you are annoyed with someone.