Reply To: Where to start
The three volumes of BFSU are an unbroken continuum of lessons that build understanding in logical, systematic steps all the way from K to 8. Hence early lessons are prerequisite to later lessons. Therefore, I recommend actually doing a quick review of Volume I, since it was several years ago, then moving through Volume II before tacking Volume III. All lessons can be addressed in an age-appropriate manner.
I hope that others who have faced the same problem will comment as to what their experience has beed.