Surfer slang (although also extends to other boardsports, and surf sports) for a young surfer. Can also be used to refer to children in general, although usually they have some association with surfing/etc.
Although usually there is no ill-meaning behind the term, it is often used to poke fun at younger surfers. It can sometimes be used as a derogatory term when younger surfers are annoying, bad or otherwise difficult.
A variation on 'grom' or 'grommet.' A nickname for a young, talented, action sports athlete. Grommie usually applies only to riders under age 15 years. Most often used in skateboarding, snowboarding, and surfing, but is quickly gaining popularity in motocross, wakeboarding, and freestyle bmx.
Did you see that grommie hit the 100' step-up? So sick.