This is an interesting case and - were it not for the fact that XFOR has a reputation for this type of bizarre behaviour - I would tend to think that there was more to it.
In fact, had the couple refused point-blank to pay they would have been on safe ground, as there is nothing in criminal law that holds a parent accountable for the actions of their offspring. Under the Children Act it might be possible to examine whether the parents were exercising due care and attention in regard to the care of their child, but any prosecution for that would almost certainly fail, even if there were anyone in CPS or the Police unwise enough to try to mount such a case. In any case, under the legislation to which Yorkie pointed the only offence which could be committed by the child would be to fail to supply its full name and address when so questioned and, because of its age, no action could be taken anyway.
The elderly couple, however, would not be quite so lucky. Once again, the most significant offence would be to fail to supply name and address when questioned; most courts, however, would be irritated were prosecutions bought against law-abiding guests of the town for a very minor infraction where significant doubt existed as to whether they even knew they'd dropped the litter.
