If you are using the DxDDP, then yes, Mickey's Back Yard BBQ is a cost effective use of your meal credits. A good rule of thumb on the DxDDP is that no matterr the mix of TS/signature/etc. as long as no more than 55-60% of your credits are used for CS, you'll save money over OOP, and if 45-55% are used for CS you'll save money over the
DDP. Of course, your mileage may vary a bit. If you are doing 66% CS, then use the DDP. Since you are using 0% CS, you should make out like a bandit.
Regarding DD9 and MBBBQ, I have been to MBBBQ, and the main attraction is that you get to dance on the floor with Mickey, (Minnie?), Goofy, Chip and Dale. Most of the children were 3-7 when I was there A princess only non-Mickey fan 9 year old might not like it. If DD9 likes Mickey and country music, then MBBBQ should be great.