It is best practice to keep children under the age of 13 in the back seat. The sensors that turn the air bags off help reduce the risk of injury, but they are not infallible.
Children who are sitting perfectly in position when an airbag goes off have a minimal risk of serious injury from the airbag; but children who are leaning forward or whose seat belts fail to keep them securely in place in case of an accident are at a much greater risk of airbag injuries than adults are. This is because their skeletons are not yet as strong as an adult's. This is true regardless of the size of the child. A 6 foot tall 11 year old is no safer in the front seat than a 4'10" 11 year old.
Depending on your state law, you may not have much choice in the matter. More and more states are enacting laws which make it illegal for children under 13 to ride in the front seat. Even if your state isn't one of them, it's something you need to keep in mind if you drive to other states with your kids.