I wouldn't do it. As PP have said, once you give Disney her birthdate, it will be in their computers forever. Which means when she is 9 1/2 Disney's record will show her as 10, to them an adult.
Also, your 2-1/2 yr. old will be able to eat for free at all buffets. So that won't cost you any more. At non-buffet restaurants your 2 DDs can share, of course, for no cost. Some servers will charge you for a 2nd drink, others may not.
As for snacks just share your total snacks amongst the family. Personally, we go for 10 nights with 3 of us and by the end of the trip we are scrambling to use all of our CS and SC. We've had the
DDP 5 times now and it's happened to us everytime. Now I will say that we do not spend all of our days at Disney. We always take a few days to explore Florida.
If you use 3 CS for your lunch you will get 3 desserts with them. Many of them are easy to carry around. I find that if we don't share a meal (which is something my DM and I do frequently) then we usually don't have room for dessert. So you could carry at least one of those extra desserts to be eaten as a snack later in the day. Also, you'll get 3 desserts with dinner. 3 desserts will be plenty for 2A & 2K to share.
Regarding sharing, you and your DH may find out that ordering 2 adult meals for each of you is more than you'll be able to eat. So at some meals, you could order 2 adult meals for the 4 of you to share, or 1A and 1K to share. Oh, and at TS your DD will get an appetizer with her meal. Adding even more food to the mix.
This year, my DD9 will be turning 10 on our trip.
For us, that mean's she's know considered an adult. I know we will have a lot of credits to use up towards the end of the trip since with what is now considered 3 adults on the DDP. I know we are going to be ordering mostly 2 meals to share. In the past the 3 of us have had many meals where we shared 1 adult meal and 1 kid's meal or 2 adult meals. We never walked away hungry.
Oh, and back to the subject of desserts using your CS credits---Flametree BBQ offers the frozen lemonade as a dessert choice. It's the full size one. Also, at the waterparks you can return later in the day to get your desserts. So, if you were at one of the waterparks and everyone wanted a dessert, you could use 3 credits for 3 of you and one of your desserts from lunch for another. (if you think you will do this I would rec. buying your lunches on 2 different receipts-that way you can use just 1 dessert instead of having to get all 3 at once.)
In closing, what your TA is suggesting is against Disney policy. Phantom children are one of the reasons Disney know insists that everyone in your group be there when you check in. It didn't take long for guests to realize by paying less than $70 for a one day kid's ticket they would get more than that in free food. So they just created a "phantom kid". When you check in now at your resorts, if everyone is not there, only the amount of guests who are there will have their DDP activated.
Since you are going in Sept. you may find that all of your eating habits change. We find that when it's hot our appetites are less.
EDITTED-Okay, so I just noticed that your other DD is only 7 months old which does change some things. However, at 7 months she proably won't be eating that much anyway. You could still share 2 adult meals amongst the 4 of you. Also, those times when your older DD is hungry, go ahead and order her the kid's meal-they are pretty reasonable.
They would probably rather you pay $69 for a park ticket than have your children eating off the buffet for free.
