I actually roleplayed this out with some players of mine back in 2E days. It is amazing how, under parental controlled conditions, fast their decendents can safely gain levels. They got hurt a lot but they didn't die. Really helps if your parents or good friend are good at summoning. "Train" them up to 4th or 5th level, put a contingency or two on them, and off they go, usually to a very successful adventuring career. With mommy and/or daddy there to pull thieir fat out of the fire, or raise them from the dead, if they weren't fast enough to save them.
So no, they aren't born with high HD, but with attentive, protective, and helpful powerful parents and family friends, they can get there rather quickly. Especially now in 3E.