I think it is unrealistic to expect writing, of any kind, from a child that young unless they are specifically gifted in that area. Two or three sentences is the most I would expect from my 7 year old son. My daughter, who is gifted in language arts, didn't start putting stories together until the age of 8 and only when I allowed her to type them instead of write. She still has illogical parts in the story due to lack of experience (ie. a girl finding the guy who caused her stable to burn down and him handing her a few bucks out of his pocket to pay for the damage), but the rest is fine.
It is for this reason, that I don't even bother teaching writing, in any form, until 4th grade. Before then, we work on solidifying their knowledge of grammar, punctuation, etc.
