DD is 8 now, and has gone through periods of eating well and widely, and periods of fussiness. (She has aspergers and some sensitivities seem to surface at times because of that, which doesn't help). And the meds she is on have a side effect of reducing her appetite - which also doesn't help.
We have always tried to get her to eat as much and as healthily as possible. She is not a great meat or fish eater at all, and doesn't eat a huge quantity generally. She is allowed some treats (biscuits, crisps, sweets etc) but also has to eat proper food, and it wouldn't be as often as a treat every day during the day. I have been happy for her to have a "cookie" (usually something like a couple of biscuits, caramel slice or scoop of ice-cream) going to bed with a glass of milk - she does need more food, and if it means she will have SOMETHING extra going to bed, I will go with that at the moment. If she is actually still hungry at that stage (when we have growth spurts especially), she gets an apple and will often eat it. I don't want to have too many rows about food so she starts refusing it, as long as we have SOME balance between good and not so good food (and at the end of the day, she needs the calories).
In the past couple of weeks, as part of preparing for back to school, I've been introducing new ideas. So together we drew up a list of dinners that she loves - there are about 8 on it. Every week, she will help me write the menu (I tend to write a rough menu for the week's dinners when doing the shopping list at the weekend) and she will be allowed to choose 2 off that list that we will all eat. But we will have at least 1 night that we will have something she doesn't normally eat (either generally just DH and I eat it, or a completely new recipe to us all) and agrees to try that. (My plan is to have something that I know she'll eat on the table that night too - tomatoes and cheese maybe - just in case).
She is also much more likely to eat a meal if she has helped to choose the recipe and/or prepare it. So she often mixes things, she's big enough now to chop, she has been whisking eggs since she was 1, and she can get out ingredients herself now too.
I have also found recently that she is more likely to put food on her plate if I put things on the table to serve ourselves. Either the full dinner (last night it was lasagne), or some side dishes (like a bowl of tomatoes, or coleslaw) - she may not finish it all (we will work on that too once we are better at eating plenty), but she is more likely to take food when it's her choice. And putting small portions on her plate rather than a large portion I would like her to eat - she is more likely to finish it, may ask for more, and there are fewer arguments about eating and she seems happier that she HAS been successful in clearing her plate.
And I've also found that DD is a grazer more than a "3 large meals" person - so putting out lots of smaller (mostly healthy food) snacks at intervals can help too. Like raisins, apples, grated cheese, carrot sticks, rice cakes, toast with nutella, etc. And drinking lots of milk.
It is hard to watch but she genuinely does not seem hungry and does eat what seems to be enough to keep her going (not growing hugely and very skinny, but Dr is watching generally and not concerned about it). So we just do our best to make sure she does get as much food as possible, and the balance (between healthy and junk) is kept as positive as possible. And not give her any issues around food that will give her bigger problems in the future.
From what you have said, it certainly looks like everything she eats is healthy and there is something from each food group. That is a good thing at least.