I think that after the initial adventure, the dog has been trying to lead her home, but she was too young to realise that, and kept walking and then maybe sleeping and the dog stayed with her. That dog could have found her own way home in minutes, but not without leaving the little girl.
The 3 year old in Australia was found 3 miles from her home, which she and the dog had covered in a day and a half. The forest in Australia has really dense undergrowth. We do not have cold enough winters to kill it off, also deadly snakes and spiders.