Someone who has an incredible ability to leap and flip, more so than would another person with the same training. Commonly a grantkins will also be able to do a handful of other things uncommonly well. This may mean that they can sing, play an instrument, draw, fight, or dance. Usually unassuming unless coerced into demonstrating these special talents.
Kid A:) That grantkins is doing mad flips and jumping over people in the other room. Can you hear what song it is that he's singing?
Kid B:) I think he wrote it himself. It's probably on you tube.