One more point...farsightedness in young children often changed at around 7 or 8 years old so it's possible that things really did change between those two visits.
And the winner is.... Dr. Chaim Cohen from Gefen Optical.
His eye exam results and prescription turned out to be very accurate and confirmed by Dr. Sun from Dr. Engel's group.
The first optometrist was really off, from 1 to 3 diopters in each eye!
Thank you all imamothers and Dr. Sasson - our pediatrician!