Andy Turner ) gurgled happily, sounding much like
they were saying :

A "child" is a person under 18.

That's one definition of the word, but not the only one.

http://dictionary.reference.com/search?q=child
You might like to check up on definition 4.

Perhaps the supermarkets should clarify?

It means child as in a youngster. Y'see it clearly doesn't have to be
your own child. A person might have their niece, nephew or grandchild
with them.

So when they say "parent and child", they don't really mean "parent and
child"?