At a signature restaraunt you can certainly pay OOP for whatever you order for your little one, even if you are paying with DxDP credits for your meal. You should take a look at the kids meal menus for the restaraunts where you will be having dinner, the kids meals tend to be much smaller than the adult meals & do not include an adult style appetizer, the kids meal appetizers are more along the lines of, celery, carrots & dip or chicken soup or a sml fruit salad.
The official line is that kids meal credits (less expensive credits) should be used to purchase meals from the kids menus, even if Disney cannot be bothered to keep track of them for you. Posting ways to get around the official policy can & will get you in trouble around here. That is probably why some people read the thread & declined to comment -- or they were looking for answers themselves.
When I did DxDP (2 Adults & 1 child) we left w/ credits still "on the table" so to speak, we just coudn't eat that much. I guess if you have a signature meal planned every night (which truly might get a bit tiresome with a three year old) you may well come close to using every last credit.