I've often done that with my own kids, once tried it with a child at my work - trying to none with them - kid took it seriously and screamed the place down. No young kids get sarcasm, but lots don't get exposed to silliness and so will believe even ludicrous stuff from parents. That child taught me well!
I think people often underestimate the role trust plays in humor (with both kids and adults). This explains why some pretty awful jokes can be funny when coming from someone you know intimately but not from a stranger, and why your kids find it funny when you suggest xyz as a consequence, but someone else's is horrified.
