Because German is the language of specific philosophers not translated or badly translated to other languages.
Besides, German literature has many jewels to be studied to this day, like "Das Parfum", by Patrick Süskind, or Göethe's works themselves. Kant and others.
That's a good argument but I still think that children are better off learning English or Chinese. Spanish too because it's one of the most spoken languages in the world.
Those languages are secondary. Because they will be in contact with them through forced coexistence.
Learning a language that holds its own treasures means that individual can resort to that knowledge, whereas others will just revolve around the same.
