Grains look like they might be good? Things like barley, pastas, oats, etc.
One of the things I've been making recently is pasta with whole barley in the sauce. It helps to cook the barley a little before throwing it in the sauce....how much depends on how you like things. If you plug it into cronometer which is excellent as david as suggested, you will see you can get 27.5g of protein from 50g of barley, 100g of noodles, and 500g of tomatoes. If you scroll down to the amino acids you will see it's kinda short of lysine though(you get for example 50% of your daily protein, but only 40% of specifically the lysine you need.)
That's no problem though because you can get that lysine from another meal. Or simply eat a little more protein than you need per day since it won't hurt you. Sunflower seeds are good for lysine though, they will meet your lysine requirement before your protein requirement. Other things like peanuts and pumpkin seeds are also good. Does your daughter like trail mix? If she were to have trail mix as a snack or part of lunch of something she'd be doing pretty well.
I quickly put together a kinda sorta meal plan based around getting protein
Breakfast, oatmeal(75g dry weight of oats)
snack - trail mix containing 25g of peanuts, 25g of sunflower seeds, and 25g of pumpkin seeds
another meal...dinner I guess? 500g tomatoes, 50g barley(dry weight), 100g pasta(dry weight)
With this you get around 1400 calories and about 60g of protein with near perfect lysine coverage and all the other essential amino acides covered no problem.
Now according to the CDC, a six year old needs about 20g
So you'd be getting triple the recommended daily protein with what I suggested and since 1400 calories might not even be enough for her you could easily get four times the amount.
In short, I think you're going to find it very easy to get enough protein for your daughter. Also for fun, try comparing eggs to the items I have suggested. You might be surprised.