Add child to German Rail Twin Pass

Do you mean have the child as the second person, or as a third? I don’t think it can be done. 

Just found a way: you add the regular adult/youth pass with children and twin pass first, then remove the adult/youth one in regular pass. However, I end up not using it because my trip is quite tricky: land at Munich, travel to Trento then back to Salzburg, I can not use the german pass on second leg without bypass Munich.