OK Louisa, I'm going to give it my best, but I'm NO expert, so I hope someone will jump in if I say anything incorrect:
First of all, it looks like you're only planning on visiting 1 park per day and not purchasing "Park Hoppers", right? (Park Hoppers give you the option of visiting more than 1 park per day, but they cost about $57 more per day - whether adult or child.) And before I go any further, Cheshire Figment has a very detailed post on everything you could ever possibly want to know about WDW tickets. It's entitled "Everything About WDW Tickets" and is a stickey at the beginning of the Theme Parks, Attractions, and Strategies Forum. There is a wealth of info there - I had to read it repeatedly to fully understand the nuances - so don't be afraid to ask questions there if you need something clarified.
So it looks like you'll be needing tickets over a 17 day period. BTW - you have a wonderful trip planned - congrats!
During that time you will need park admission on 7 different days:
1. Sept 15 - AK
2. Sept 16 - MK (including CRT)
3. Sept 18 - HS
4. Sept 19 - EP
5. Sept 20 - MK for MNSSHP *You will need to have a ticket for admission into the MK as well as your ticket for MNSSHP*
6. Sept 29 - MK for MNSSHP * See above*
7. Oct 1 - MK for Terrace Dessert Party - again, you will need an adm tix to MK
The only problem is, tickets are only good for 14 days after the first use, (unless you purchase the "No Expiration" option which is an additional ~$190 per adult & ~$152 per child) so you may want to try switching some of your plans around, if possible. That is, if you use your first ticket on Sept 15, the last day you could use the rest of your tickets would be on Sept 29, if I'm not mistaken.
Is there any chance you could visit AK while you're at BLT? Then your first use day wouldn't be until Sept 16 - stretching your valid days until Sept 30. That would also entail moving the Terrace Dessert Party back, if possible. Could you move that to one of your BLT days as well? It's a short walk to MK from the Contemporary - about 10 minutes, I think.
I can certainly see why you planned the parks the way you did, but I'm not sure that you realised the tickets were only good for 14 days from first use...
BTW, if you haven't purchased your tickets as part of a package, they are a bit cheaper thru
Undercover Tourist. You'd have to sign up for a newsletter at Mousesavers.com and in that is a link to Undercover Tourist.
I've probably made this as clear as mud, so please ask and I'm sure the experts on here will chime in for you.