"But neither of you can seriously be advocating that this should have been performed in Dutch, are you?"
No I just think they shouldn't sound like they just graduated from RADA. And the Franks were from Germany, so the speech patterns of Anne's parents would have influenced her to have more of a German sound, which has a more gutteral (harsher?) sound than British.
In the movie "The Hiding Place" which is a parallel to the Anne Frank story, Julie Harris did attempt a Dutch accent.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.