First, if you only want a calendar of birth date anniversaries, I would suggest you look at John Cardinal's program "On This Day".
It can make a calendar from your TMG project of events throughout the year. You can filter for BMDB ((birth, marriage, divorce, death, burial) events, and restrict to Primary events. This might directly do what you are desiring.
However if you want a full chronological list of birth dates you need to do something like Terry mentioned. Terry is correct that the List of People report has no options to format the date, so sorting on a column of the month output with alpha characters does not sort chronologically. However, the LOP does (somewhat) abide by the Program Preferences for the date format. You could (temporarily) set your "Preferences / Program Options / General / Date format" to "yyyy.mm.dd". The LOP output column of "Birth Group* tag; Date" will use that format, and you can choose to sort the report by that column which should produce (approximately) chronological results. Unfortunately, the LOP output column of "Birth Group* tag; Month" expects the input to be alpha characters, and thus the report outputs a value of zero for all months.
However, the List of Events report does abide by the Program Options for the date format. Thus one could perform a two step process using a temporary Flag. Run a List of People report like Terry suggested to set a Flag to identify the descendants which you want. Then run a List of Events report filtered for Principal1 having that Flag value, and Tag Type Group is Birth. Sort the Output Columns on (now with numeric values): 1=Date - Year, 2=Date - Month, 3=Date - Day. Add any other output columns such as the Prin1 name and/or ID and the full Date value.
That will give you a full chronological list of birth dates of these descendants.
Hope this gives you ideas,