I've heard of hamsters living with tumors, but the only way you'll be able to get a real prognosis is to take her to a vet. Hamsters only live 1-2 years (sometimes more, but this isn't the norm). I know that it's really hard losing a pet
Have you thought about getting them pets that live a bit longer, like guinea pigs or rabbits? Maybe introducing another hamster will help ease the pain.
Dwarf hamsters are social and you can have more than one in a cage. Hamsters can go down hill quickly because they hide pain or injuries well. So she may seem fine one day and then pass away the next. It's hard to tell without having a vet examine her. Just make sure she eats and drinks. There is not much else you can do without a vet.
This could be a good opportunity to teach about life cycles, religion (such as afterlife or w/e your beliefs are), etc. Death is a life lesson everyone has to come to grips with. This doesn't make it any easier, but it's good they are dealing with losing pets now and will be prepared for when they lose a human friend/family member. So, it's really hard, but it does help.
