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.

Trento to Salzburg can be be done directly. Via Munich would be a big detour. What's your travel date?

With a German rail pass you could do it with a switch in Rosenheim right? You can't use the Innsbruck-Salzburg Railjets though.

Ah yes indeed, that would be a good solution.