Kassie, welcome to cruise-chat.
Although I feel your anger, this has been a policy of ALL cruiselines for a few years now. Any children who are in diapers or swim diapers are not allowed in the pools whether they are adult or family pools.
This rule went into effect due to the possiblity of disease. I know personally I do not want to be in a pool with kids wetting and pooping in the pool. E-coli is a very real disease. How would you have felt if you were in the pool and a "poop" came floating by you?? YOu would be very upset. The reason for this rule is for sanitation purposes.
I feel your anger, but if the rule was broke for your granddaughter and maybe another 100 or so toddlers and babies, then the water could carry contaminations. I personally have to agree with this rule.