Cyberpunk 2077's Night City is full of people for you to beat up, including gang members, cops and corporate douchebags—there are limits, though, so you won't be brawling with everyone. Children, for instance, will be protected from your murderous rages, along with story-important NPCs.
"You won't be able to attack children or NPCs connected with the plot," reads the reply, "but other than that you can be aggressive towards most people you meet."
CDPR confirmed that this is the case in an email, telling me that there will also be areas where aggressive behaviour will come with additional consequences. If you try to start shit in gang or police-controlled territory, they'll come down on you.
It makes sense, though I'm also a fan of the Morrowind method, where everyone was fair game, even if it meant you'd sabotaged your chances of finishing your adventure by killing off someone you'll need to interact with later. But then Bethesda didn't have to worry about players slaughtering infants—that wasn't a problem until Skyrim, which had invincible kids who were unfortunately rendered mortal by modders.
Luckily for Night City's kids, Cyberpunk 2077 won't have modding support at launch, so they're safe for now.