Different scans involve different amounts of radiation. With a very low dose X-ray, such as a single chest X-ray, the dose of radiation is roughly the same as you would receive from the general environment over about a week, or from taking a long-distance, international plane flight. Other procedures — such as CT scans or X-rays of the spine — provide a larger dose of radiation and therefore have a greater, but still small, risk.
The risk of harmful effects from having X-rays and related scans usually only become significant after a person has had a substantial number of high radiation-dose scans. Children may be more sensitive to the effects of radiation and therefore be at greater risk.