My brother, at the age of about 6, having been bought a new pair of cream jeans (what was my mum thinking?!)and been told that they're for best and not for playing outside in, came in with one leg rolled up to the thigh and one up to the knee and swore blind - even as my mum was unrolling them - that he absolutely positively hadn't been on his bike in his new jeans and that the big black marks on them weren't oil from the chain. On another occasion, when asked what he'd done at school that day, told my parents that his teacher (in her late 50s) had stood on her head for an hour. Legend.
When I was at a friend's house the other day, her son (3yrs) proudly announced, "I did a wee on the train." Of course I thought he meant that he had weed while riding on a train, but he'd actually weed onto a toy train! Later that day he actually weed on his 1-yr-old sister. So the weeing on sibling story may actually be more plausible than it sounds.
My uncle wrote a story at school about how his dad (my grandad) went to the pub and punched a man on the nose. Total fiction. Cue my poor grandmother dragged into school with regards to this random act of violence.
My grandad was a police man so unlikely to have done this in his time off
I bit the new velvet curtains in the lounge (no idea why) and blamed it on my sister. Even after the subsequent tooth match up I got away with it because my front teeth had fallen out between the crime and the investigation. My parents still don't know
I know it's not a childrens lie, but I've heard of a headteacher who told a bunch of 11-year-olds that the furry hat she wears for playground duty in the winter, is made from her cat. And they believed her - it's a legend in the school, apparently.