I suggest you read some trail journals written by (or watch video blogs) and see what hikers say about what they carry foodwise and how much they eat. You don't say how long this hike will be, though you imply it is more than just a long weekend. 

Generally speaking, and YMMV, hikers on the Appalachian Trail have suppressed appetite during the first 1-2 ish weeks of their hikes. Their example should serve as a good guide no matter where you might be hiking. Many new thru-hikers overpack their food bags and arrive at the first resupply opportunity with meals still to go.

Then "hiker hunger" tends to set in. Typically thru-hikers cannot comfortably carry enough daily calories on the AT, but resupply options are plentiful over 99% of that trail. They "tank up" at town stops or road crossings to help make up the difference. Once thru-hikers have put hundreds of miles under their belts, they turn into machines into which they pour calories and out come miles. I know very few who count their calories. They just eat. 

But for your daughter, more care definitely required. One popular method of providing enough calories is the breakfast shake--Carnation mix, with add-ins as desired, sometimes mixed the night before. Olive oil, a very calorie-dense food, to mix with dinners. Parmesan cheese, same thing. Peanut butter. A little research on hiker foods will lead you all those places and more and include discussions like "to eat candy bars or restrict carbs/sugars" and "avoid processed foods vs. eat whatever, it's just fuel." 

Bottom line: IMHO carry 1-2 pounds of food per person per day. In the early days of your hike, I bet the challenge will be wanting to eat. When hiking I find I move away from "three square meals a day" and more toward eating a little something every hour or two during the hiking phase of the day with one substantial meal once the miles are done. After you and your daughter have hiked a week or two or three you'll know what works for you. Big hot breakfast followed by snacks? A 2-3 hour break in midday in which the "dinner" is prepared and eaten, followed by a rest before hiking on? 

Hope this is a good place to start. . . .