The thing is I have children of that age and believe me if my kids did such behaviour I'll be the first to give a clip round the ear, but I can't do that in a business as it will only invite more trouble than a solution for my mates business
unfortunately, kids behaviour will be a product of their parents, environment and piers
as you are not in a position of authority to discipline the kids, and they will not recognise you as an authority figure, any such action you would take would only be teaching them violence and aggression to solve violence. for all you know, the parents way of dealing with them may be to smack them round the head and you are seeing the results of this.
it is one thing to apply some mechanism of discipline (and I don’t just mean smacking / clip round the ear) to your own children but that has to be in the whole context of their environment, safety net and parental reasoning and teaching.
If you actually managed to sit down with them and ask them what they wanted, why they were here and doing what they were, it may take you a while to get passed the dunno just bored or cause we can answers.
a clip round the ear may have worked pre/post WWII but that was only because the kids recognised the authority in the adult and understood it. it doesn't work in our society like that anymore.
they may be known to the police already so work with the police.