The underlying problem still remains though. How can the shop deal with misbehaving children if the parents refuse to do anything about it? (By refuse I mean angrily tell the staff to mind their own business usually in swear words accompanied by threats).
Ask them to leave, and ban them from the shop. If they refuse to leave they can be forcibly removed (with reasonable force, but slightly risky) or the police can be called (most people will leave if they know the police have been called anyway).
If stuff is broken due to the parent's recklessness then it could be argued that it's criminal damage, and the police could be called (again, the mentioning of police could persuade them to pay up) and if necessary they could be detained until the police arrive (which could be somewhat risky again in terms of police etc).
Whilst not the same environment, I worked in security at Tesco last year and we would ask people who were abusive to leave, and then ban them if necessary (usually followed up with a letter delivered by the police).
People were also detained for shoplifting and criminal damage (usually damaging security tags) until the police arrived.
However most breakages were seen as "one of those things" and even if a customer broke a product after paying for it on the way to their car, a replacement would be offered free of charge.
One thing I did find was that whilst dealing with the police, if you were calm and seemed reasonable, they took your side more often, especially if you were the one to phone them and outlined everything step-by-step.
This was even more so if the other party was still angry and came across as irrational.