This is essentially a boy meets girl story, and you’re trying to exploit the Holocaust as a backdrop in order to increase the stakes, unfortunately to ill effect.

Here are a few reasons why:
The added complication of the German girl being Hitler’s daughter is not only a convenient writing add-on but also too big a leap of logic, which subsequently puts the suspension of disbelief under threat.
I simply can’t think of many ways to rectify this in the current concept as it is presented in the logline. Therefore, I believe you would do well to rethink this concept from the ground up, one such way of thinking (as previously suggested) could be to tell the story from the girl’s point of view instead of the boy’s – the story needs to change if you want to make it work.