Couple of things here.
First, But if that happens, arrest them and reveal their identity at that point.
This is way easier said than done. Protests are typically large groups, and the larger they get the greater the chance for criminal activity. Beyond that, when the start to grow beyond a given area and start to expand (BLM or Occupy) there are people all over the place.
Even IF they are not huge and spread out, they can easily scatter when the police show up, if they are even around by the time the police arrive. They usually aren't.
Going with the angle of let of where masks until a crime is committed glosses over the reality of the situation. It's a nice tidy little idea, but it just doesn't work.
As far as your children or grand children go, anyone can get on the slippery slope of for the children. I could come up with a complete list of why we should abolish the entire Bill of Rights, "for the children".
It's quiet possible future generations will face dangers given laws at they sit today, in fact, it's nearly guaranteed. However, we can not govern the country based on those fears alone, be they legitimate or propaganda. It often leads to terrible laws and usurpation of rights. Sometimes, and frankly more time than not, in the extreme.
Want clear proof? War on Drugs and Patriot Act.