Someone is trying to milk you. If it was genuine case then the mother would have spoken to you directly in an adult fashion. Her kid might also have told her a different cooked up story. Either way the mum should have spoken to you.

They want CHF400 for the jacket? Fine! You want CHF1000 for the psychological trauma your child suffered as a result of the punch from the other kid. Send them a bill for CHF600. If you are in a good mood then offer 5% discount.

On a side note if your child had ripped the jacket in an unprovoked attack then full cost of the jacket would have been fair rather than repair.