Vaccines are usually tested in adults first, to avoid exposing children who are still developing and growing. COVID-19 has also been a more serious and dangerous disease among older populations. Now that the vaccines have been determined to be safe for adults, they are being studied in children. Once those studies have been completed, we should know more and guidelines will be developed.
In the meantime, make sure children continue to physical distance from others, clean their hands frequently, sneeze and cough into their elbow and wear a mask if age appropriate.