I had a book I loved as a child but I can't remember much about it. The drawings were similar to this style: http://www.amazon.co.uk/Complete-Brambly-Hedge-Jill-Barklem/dp/0001983679/ref=cm_lmf_tit_12

It was about a girl that had a pointed red fuzzy looking hat, and she lived in the forest, with mice? I remember mushrooms. Maybe she was tiny? It was probably 70's or 80's. I don't think it was Thumbelina.