Ah, kids. They're so cute... so innocent... so gullible. This trope is for any older character (such as a parent or older sibling) that takes advantage of a young child's naivete in order to tell them outrageous lies, either for their own amusement, or to avoid a tricky question that they can't answer.
Often, they will be caught out, usually when the youngster repeats the new "fact", along with the name of the clever person who told it to them.
See also: Little Known Facts
This Australian ad
for Bigpond Broadband
- This is the premise behind the book Great Lies To Tell Small Kids.