What does it mean when Anne Frank was described as "a symbol for the lost promise of the children who died in the Holocaust"?

I’m doing an English lesson on the Holocaust, and on one of the questions, I had to answer what the phrase meant. I’ve tried looking up what “lost promise” means, but none of the answers I’ve found fit the context, and I’m not sure how to answer. The only answer I can find is when one of two parties breaks a promise, the other party has lost the promise, therefore it is a lost promise. I can’t think of any other meanings, much less for “…a symbol for the lost promise of the children…”
I know someone else was asking what “lost promise” meant in the context of Nickelback lyrics, which is where I found the above meaning, but if anyone can find anything different I’d be grateful. 🙂