The danger in skiing (assuming that a helmet is worn) can also be from tumbling type falls and falls where you lose an edge and fall backwards into the slope. I agree with Mark that common sense is the best approach to living with concussion sensitivity.
Contact sports (football, hockey, lacrosse) are always going to worry you and with good reason.
I'm sure you are aware that it's not always the severity of the bump to the head that generates the most problems. Too many of us have received our concussions from normal everyday events that went bad, falls, etc.
I know it's hard when it's your child, but it's also hard to try and raise them in a bubble.