What to do if a kid hits a teenager, or an adult?

[LinkxPeach]

Some kids are assaulting teenagers, and adults who did nothing to them. Now, if a teenager, or adult hits them back. People will try to make the teenager, or adult look bad, and try to punish them. Kids think that life isn’t fair. But, they’re the ones being unfair because they picking on people who do they think not gonna do anything to them, and it’s illegal for a teenager, or adult, and cowardly for an adult to hit a kid.

[TommyMountainFigure]

Yes, but it isn't illegal to restrain them in self defence. If a kid lashes out at you, grab their leading arm and twist it around their back so they can't hit you. Then either you with your free hand or someone else who might be close by, ask them to call the police.
You are well within your rights to perform a citizen's arrest using minimal force (At least you are in the UK)