Now, if you are not going during
free dining and would opt to pay the $30 a DAY for your child to eat the adult meals...that's a little different. But this is FREE, and a 7 year old is entitled to a FREE 7 year old plan.
Just think - if everyone going during free dining did this (I'm only getting 1 day park passes - so I'm sure I could pay about $20 total to get the adult plan for my 3 and 5 year olds) - and everyone had free adult meals for all their kids....I'm betting that Free dining would not be around in the future.
We're doing that for DS, who will be about 6 weeks shy of 10 when we go, but we aren't going during free dining, which may explain why the CM we talked to was more than willing to do it this way. He's an adventurous eater, something of a foodie (as much as a 9 yo can be!), and easily eats more than I do, so I'm sure we'll get our money's worth out of the adult plan for him. We did have to have him entered into the system as 10, because otherwise the computer automatically counts him as a child.
I don't think I'd bother making a 7 year old an adult unless he's a huge eater. When DS was 7, he was fine with sharing our meals in addition to his kids' meal, and at the buffets, the kids can have anything anyway. DS's favorite meal was Boma and he never even looked at the 'kids' offerings.
The dining plan is so much food that there is plenty for him to try a few bites of what everyone else is having, and the kids' meals at most of the table service places don't look too bad anyway.
I agree with both of these posts, 100%. I think the ethical sticking point here is that the dining plan is
free. If it wasn't free, and a parent is willing to pay the adult price for their child's ticket
and the extra $28 per day for the adult dining plan (as Colleen is doing for her almost 10 year old), then I think most folks wouldn't see a problem with it -- especially if you are totally upfront with the CM that you are "aging" your child specifically so you can purchase the adult plan for them.
I'm really very surprised that Disney Dining would permit this during free dining. Here's why:
Family of 4, 2 A and 2 C aged 4 and 6. Disney trip is for 7 nights. For free dining, they should get 14 A TS and 14 A CS, and 14 C TS and 14 C CS. If the parents "age" their children to get the adult's plan free, it costs $34 per child for the whole week for the tickets (I believe that is what most people have reported). That's $68. However, now the family has 28 A TS and CS. The difference in value for the A credits vs the C plans is $28 x 14 -- $392. That's $392
in addition to the original value of the free dining, $700. Will those little kids really order and eat an entire adult CS meal, and an adult appetizer, entree, and dessert every day? Probably not. What will the family do with the extra TS credits? Probably use them for signature meals and dinner shows, while paying the much lower price for the children OOP.
However, whether or not it makes financial sense for Disney, and ethical questions aside, many people have reported that Disney Dining CM's have allowed them to do this during free dining. That can always change

Last year people said CM's were allowing and even encouraging them to bank kids' TS credits to use for adult meals, but that practice was eventually stopped. When Lori called Disney Dining, the CM put her on hold to check, then told her she wasn't able to "age" her child to get the adult's dining plan for free; Laura was also put on hold, then told she
could do this. One of those CM's was mistaken, and
supposedly they both checked with their supervisors when they placed Lori and Laura on hold, which would mean one of those supervisors was wrong. I work in a call center -- it's very possible that one of those two CM's just checked with the person sitting next to them, or tried to find the answer in writing and couldn't.
As the PP's mentioned, there is so much food on the Dining Plan that there is plenty to share with your children, and at virtually all character meals and at all buffets, the kids would have all the same choices as the adults anyway, and can eat as much as they want.