If you have ever wandered into a store filled with rows of breakable glassware with young children in tow, you understand what anxiety is.
Sometimes such shops will post the sign "you break it, you bought it", So assuming you (or your kids) go in and knock something off the shelf, did you in fact buy it?
Contract Law
Contract law has a concept called unilateral contracts. Unilateral contracts where the terms are not agreed to by both parties are not binding, so it is unlikely that the if you break it you bought rule is an enforceable contractual term.
Negligence
Contract law is not the end of the story, however. If you were careless and you broke something in circumstances where the courts determine that you have a duty of care, then your negligence could cause you to pay the price of the item, and perhaps a sum for paying to have it cleaned up. You might be able to point blame back at the shopkeeper, so the outcome of such a case would be far from certain.
Cost
Finally, costs and practicality would come into play. If you knock over a $20 glass plate, it would not make sense to spend thousands of dollars on a lawyer. If you knock over a priceless urn, then perhaps you might be sued.
