Anyone coming from lower altitudes to 8,000 feet or more may experience headaches, unusual fatigue, nausea, loss of appetite, difficulty sleeping, and lassitude. Expect, also, that you'll do some heavy breathing, even after you acclimatize. These physiological responses are discomforting, but not serious.
As high as they can climb. Kids are sturdier than we think and scientific data supports this. Studies suggest that young hikers are not more susceptible to altitude sickness than the rest of us; indeed, many children travel to resorts at high elevations without complications.