Hi
Think it is just "looking around" and comparing prices... I have personally booked cruises through both:
-
http://www.dreamsunlimitedtravel.com and
-
http://www.vacationstogo.com/?dom=vacationstogo.co.uk&ref=none
Got excellent prices and service through both (although I am not a demanding customer...) and wouldn't hesitate to use either again.
I have never booked direct through Disney, but have booked direct with other cruise lines... wouldn't hesitate to book direct either.
Would just look around and find the best price/deal (as mentioned before, sometimes you get "shipboard credit" offers... where the agent will give you $50/$100/$200 credit for your account on the ship).
As others have mentioned, it is worth giving Virgin and the other package holiday operators a "try" too - see if they are competitive.
Think that the 3 night stay in I-drive, 4 night cruise, then 7 night stay on I-drive SHOULD work out well for you and your family: time to aclimitise and reset your body clocks before going off on a cruise, then you have 7 days at Disney to look forward after the cruise too!
Disney cruises TEND to be more expensive than other cruises (i.e.
Royal Caribbean) - like all things Disney tend to be. BUT sounds as though your son is at the perfect age to enjoy the specialness of a
Disney cruise 
(husband and I enjoyed Disney cruise, but wont cruise them again until we have a DD/DS of the right age... we have no kids at the moment...).
Boo