Ahoy Ashlee!
The Alaskan itinerary looks absolutely amazing, and I agree that Disney Cruise Line is the best choice for your family! The Port Adventure team has gone above and beyond to create Disney exclusive experiences with families in mind so your onshore memories will be extraordinary! Couple that with all your onboard fun and you have the perfect recipe for a magical vacation filled with pixie dust!
No matter which month you choose, you will be in awe of the spectacular scenery and unique wildlife. Both May and September have similar weather which can be very unpredictable during the summer time. In my research, I have found that the weather really doesn't seem to matter because the majestic views are stunning at all times. With that being said, it does rain a little less in May and you may be lucky enough to see some animal migration. In September, you will have less daylight which isn't a bad thing since it may be dark enough to observe the Northern Lights.
I hope I've given you some things to think about, Ashlee. My advice is to check out the
Alaskan Video Series with Cruise Mom Andi. It is a great sneak peak at what you can expect on this phenomenal cruise journey!
If you have any other questions, please let me know! I'm always glad to help!
Have a magical day and happy planning!
Nancy