Add a sex task to the litters, just like the butcher or weigh. Mark the date they were sexed and number of male, female, and unknown. Numbers assigned to whole litter or gender to individual kits.
8 kits in litter
Sexed at 2 days
Or have the sex assigned to each kit using check boxes instead of editing each kits profile.
(Similar to when you butcher them)
Maybe have 3 check boxes for each kit in the assigning sex list/option. One for male, female and unknown/uncertain. Only one checkbox may be selected