Generally, German cities are incredibly safe, and whether you choose the German capital or other cities, you should have a great time there. Not only for tourism, but living, raising your children, and building a life in the major cities of Germany is going to be a treat.
Like everywhere else, some cities in Germany are safer than others. Like almost everywhere else in Europe, the larger cities like Berlin, Frankfurt, and Munich generally experience more crime per capita while crimes rarely happen in a small city. That being said, even the German city with the highest crime rate is still exceptionally safe.