It is allowed to carry children who are over the age of 18 in the front seat of a passenger car that is not equipped with a child restraint device:

Erklärung

Without a child restraint device, front-seat travel under the stated rule begins once a child has turned 12 and can be secured by the vehicle's regular restraint system. Being 10 or 11 does not cross that threshold, so either younger age leaves the child within the restraint-device requirement.