As a father of two teenaged boys who drive fast cars, I'd much rather hear it from them than someone else.
Case in point. One of my best buddies has an 18 year old son. He had a ricered up Plymouth Laser and bragged to my boys several times about street racing and how fast his car was (in all of it's 150 hp glory...). My sons told me what the kid said - they don't street race, we race on SCCA autocross tracks.
I talked to his dad - he limited his use of his car, citing the unusual tire wear present and rubber chunks in the fron wheel wells, rather than diming out my sons.
The kid blew up his clutch twice and his engine once. Dad paid to have it fixed - not something I would have ever considered.
Anyhoo, the kid's getting ready to go to college (local), so mom helps him find a 2001 Audi TT Quattro with engine mods and 19" wheels....for $15k - bear in mind, he wouldn't even change his own oil or take care of his previous car.
A week or so after getting the Audi, he bragged to my sons that he got a ticket for having a speed contest on Rt 309 (4 lane hwy) with two of his buddies - he explained that he'd just pay the $350 ticket and his dad would never know...
You know the rest of the story - my buddy isn't an ebay guy, but I am - I sold his car on ebay for his dad.
If he would have told his dad, he would have gotten his ass chewed, but he'd still have his car.