Hello,
The Grimm's fairy tales are folk tales of Germany. The Grimm brothers did the great work of collecting them from many sources. The first collection (if I am right) was a simple group only with little 'touches' in language and expressions. After that, a new edition followed, in which they 'cleared' the docs from some violence or other 'things' that may hurt children and they added (sometimes) precepts, for the good of children again. However, they respected the folk tales in a great manner. That's all I know.
Now, I think it's wrong to say that 'their aim was to create a national identity for the German people'. The truth (as I know it) is that they wanted to collect all the german fairy tales and tried to exclude tales from different nations from their collection.
A link for their life (from google) is
here.