(whoever decided a 10 year old would eat adult portions?)
It's not really an "adult" versus "child" thing. There are two prices for the Dining Plan: The $38.99 "regular" price, and a discounted price of $10.99 for young children "3-9 years old". Regarding portion sizes, I think the issue is that many 10 year olds won't be happy with young child portions. Disney has to draw the line somewhere, and one objective was to synchronize the ages between dining and admission.
There will be 3 adults ... and 1 child on our plan. From everything I've read, the consensus seems to be that with the
DDP there is more than enough food. This being said, could we go into a restaurant and order only 3 adult meals and share our meals or our appetizers with the child?
After January 1, you should be allowed to do that, because there will be measures in place to
prevent you from using the unused child credits for adult meals later in your trip. Until those measures are in place and all the CMs know about them, you may be prevented from not using a credit for a child with you at a meal, when there are child credits as part of your package (which is clearly indicated on your room key/room charge card).
I have no idea what we would do with the leftover child credits at the end but it seems like a waste to knowingly order more food than we will be able to eat.
I really wouldn't worry about it. Between the various bits of the child's meal, there may be one bit that the child actually wants.
Maybe I could then use her credits as snack credits to get all the junk food I know she will want?
Unfortunately, TS credits cannot be used for snacks. Under certain circumstances, you could convert a few TS credits to CS credits, but after January 1, you may not be allowed to
just convert child credits. We really don't know all the intricacies of the new information system they're implementing, so until we do there is really no way to know what you'd encounter.