Most large chain pet stores source their 'stock' from rodent mills, which are practically cages full of hamsters, being bred over and over again, no care for genetics or health concerns.
They also sell very inappropriate products, like too small cages, but they get away with saying that they are only temporary cages up to around 12 weeks of age, when they sell hamsters around 6-10 weeks, so would only last them a few weeks if that! The RSPCA usually recommend a cage that is 80cm by 50cm or larger, the only suitable cage that Pets at Home currently sell is the Savic Plaza. The hamster heaven claims to be suitable size at 80 by 50 but it tapers in at the bottom, so actually works out much smaller, the tubes and penthouse are known for escapees and Syrian hamsters have died in tubes that size, not to mention the front door, so many escapee reports, it needs to be closed on the second notch.
They also sell fluffy bedding, which has caused many deaths, toys that feel like sand but are coloured sawdust and encourages them to chew them, but can cause blockages. I was also told that they didn't need a sand bath, yet I think they are pretty essential now, after years of experience with hamsters.
Wood shavings are also hugely debated for many reasons, paper based bedding like Carefresh or Kaytee clean and cosy is much healthier and better for them.
Not just complaining about Pets at Home products! Most pet stores are the same, they don't even sell quiet wheels of a suitable size (dwarfs minimum is 8 inches and Syrians need a 12 inch wheel)
Dwarfs are highly prone to illness, eye problems and diabetes being two main ones, this is probably because most of them (I think all in the UK now) are actually hybrid dwarf hamsters, also known as Russian dwarfs. They are a hybrid of Winter whites and Campbell dwarfs.
Cleaning them out fully weekly can be extremely stressful, I don't think it was anything you have done though, but you can spot clean their poop and sand bath daily to weekly, but with the correct size cage and paper based substrate and bedding, you should only be cleaning them around every 4-6 weeks, otherwise they stress so much and can smell more as they will usually scent mark much more.
I am going to wrap it up now, sorry for the long post! I'm way too passionate when it comes to animals, especially hamsters and rabbits as they are so misunderstood and mistreated by many people and most pet store staff aren't very helpful either, although you do get the odd lovely knowledgable staff member of course!
There are so many Facebook groups to help learn from too if you get another hamster. I hope your daughter is okay, it isn't likely anything she has done, just bad luck and bad genetics, I'd look for an unwanted hamster, rescue hamster or if you can find an ethical breeder (which are very hard to come by!) Please don't support rodent mills.