Interesting dilemma. I have a question and a few preliminary thoughts.
Question -
In addition to touring the school, did each of your children have an opportunity to shadow for a day, or half-day?
Preliminary Thoughts -
Since you believe that this year's K5 experience is a uniquely good "fit" which will most likely not repeat for DS next year or thereafter... it occurs to me that the best route may be to carefully explain this... so that the child understands that
both siblings will find the opportunities at the new school to be a better "fit", ongoing. Hopefully DS5 will consider his older sister's experiences, and see that he is the lucky one, not having to suffer at age 6, 7, 8 until you found the better "fit" of the new school.
Kids need both appropriate academic challenge, and intellectual peers. If this school offers both...
