My daughter had similar issues at that age. My daughter had the binder on the side as well. She was also taught to properly keyboard at that age.
Maybe you could work with the OT on finding a seating a solution.
http://www.ballsnbands.com/childrens_exercise_ball_chair.htmlThis seat seems to be an improvement compared to the one we had which was just a ball that sat on the floor with 3 tiny feet.
When she was in school the teacher let her stand since she was between sitting and standing and that habit contributed to falling out of the seat or flipping it. She was also the size of a three year old so the tables were too high for her.
When they are younger it is easier to O.T. weak areas.
