The organic crumbles is good food but if we feed them anything besides their crumbles they need grit and when tiny chick grit is put in a small container next to their feed. Not saying this is what caused her to die it's just something not everyone knows about when just starting with ducklings, it's also a good idea to make sure they have enough niacin for strong legs most of us use Nutritional yeast sprinkled over their feed 1 Tablespoon to each cup of feed we give them this should be done daily until at least 10-12 weeks old.
I am very sorry this happened and your daughter is heartbroken I do hope you all will try again Many places have ducklings for sale right now and if you decide to try again to raise more please consider at least 3 that way if something happens again and you lose one the one left will still have a buddy. We have a lot of good information here on BYC for all stages of growth and many other things so come back and ask and we'll point you in the right direction.
Most of us have lost ducklings and on up to old age and it is very hard to lose one. We understand