That's a known Arics bug. Best way to fix it right now is to go edit your "files\scripts\person\person.gd" file.
Search for
rvar = ' (' || race_display || ')'
and replace it with
rvar = ' (' + race_display + ')'
and that will fix mixed children being displayed improperly. Alternatively you could edit your save file and change all problematic characters from mixed race to pure blooded.