Yes your understanding is right. The standard time for processing a file is 6months...however its now taking longer than 6months on average. So it is very likely that your old medicals will expire by the time you add your child and get youe visa. A new medical or extendes medical is the only way in that case.
But if you can provide an Xray...the kind done with a protective shield...your file will be processrd without any delay.
