Your credits are pooled on your key - and the key says how many adults and how many children are on your plan - so yes, when you all eat together, they will know that her credits are child credits.
When you purchase the dining plan, they tell you that a child MUST order from the children's menu, when one is available. Unfortunately - their children's menus assume that just because you are young, and may need smaller portions, that your taste buds have frozen somewhere between mac & cheese and pb&j.
At the start of the dining plan, the rules were a little more lax - and it truly was magic YOUR way dining. You could pay for the kids' meal, and use the extra credits towards a signature meal, etc. Although they have not recoded the program so that the computer can tell how many adult vs child credits have been used - the restaurants, especially some of the sit downs, are being more insistent - for example - if you want to use credit for anyone in your party, you must use them for all. I personally haven't encountered this.
We first used the dining plan when it was offered for free in September. There were five of us - 2 adults, 2 teens, and 1 child (8 turning 9 during the trip). While we did not use kids' credits for adult meals, we did use 3-4 credits per table service meal, and ordered extra drinks and maybe an entree for the ones not eating the full credit at that particular meal. We did this as we could NOT eat 4 adult appetizers and still eat our entrees and desserts! We also had more TS meals planned for this birthday trip than we had credits.
We had no trouble doing this until our last night, when we were at the 50's PT. Before we ordered, we explained to the waitress that we only had 3 credits left, as it was our last meal, and we would be ordering some items to pay cash. She said that would be great - We ordered 3 appetizers to share, then each ordered an entree - and asked that the 3 more expensive ones be put on the dining plan, and the kids' meal and the lowest priced adult meal be put on the cash ticket. She had no problem with this. Then for dessert - we just ordered the 3 dining plan desserts to share - and at that time - she wanted to know if these 3 credits were all adult credits - which they were! We had used our child TS credits for PSB, CRT, Concourse Steakhouse, and Chef Mickey's - which were better deals for the Child meal. They are all over $10 per kid, rather than the $5.99 meal she chose at the 50's.
Another strategy we used was to have my youngest order the mac & cheese (which my middle teen LOVES) and have my teen order the salmon or whatever meal my little one wanted - she loves to eat REAL food. After the food was delivered - they'd switch plates.
I am taking just my 9 year old with me to Mousefest - and we've purchased the dining plan. I'm interested to see how this works. One counter service meal will definitely feed the two of us - I'm not that big an eater - so we plan on doing some sharing. I planned on having her eat her CS meal at breakfast, while I have my coffee from a refillable mug, then sharing my CS meal at lunch. I hope they will allow this. Then we will each purchase dinner at the TS meal.
Good luck with your dilemma - I'm sure my 9 year old will be eating most of my meals!