Oh good grief, carla, DD1 had a friend like your DD when she was at primary school and her mother voluntarily gave her 2 mars bars and 2 cans of coke in every packed lunch - she was the size of a house and her mother used to nag her and complain about it and say in front of her how unattractive she was! (She was adopted and mother, father and adopted elder brother were all naturally skinny, so restricting diet was a problem for mother, who blamed it all on foster mother from 0-6weeks who overfed her, allegedly.)
Don't give her the fattening stuff!!
End of story.
You are the mother, you are the boss, you decide what she gets in her lunchbox and if she doesn't eat it and comes home starving, tough - tell her she can eat it as an after-school snack instead.
DS2 is 12, about 5'2 and 8½ stone and slightly podgy, and only has for lunch 1 peanut butter sandwich, 1 apple and 1 frube, with fresh apple-and-mango juice in his bottle. He comes home quite hungry and has a banana and some bread or toast and maybe another apple, and that does him for c. 3 hours till teatime. (I suspect he has something chocolatey at school mind you but he is at secondary school and I can't do much about that...)