I am glad that your granddaughters’ doctor found the polyps early. I was diagnosed with FAP and had surgery when I was 35, so I cannot relate well to some of the issues an 11 year old will need to deal with for this surgery. Since this is a hereditary condition, other blood relatives should be tested for FAP. I had a DNA test for this disease run at Mayo Clinic and my two sisters and two daughters were tested and found free of this condition. I suggest that your granddaughters have a surgeon who is well experienced in this procedure and has a good track record. Cleveland Clinic comes to mind as a highly rated place for jpouch surgery. A surgeon who has experience with pediatric patients would be a plus.
They will need to follow the surgeon’s recommendations carefully regarding diet and physical activities and be patient while they adjust to their new pouches. Afterwards, they will need regular scope exams to determine if any more polyps have developed. I wish you all the best of luck with the surgery.