Hehe, it's subtle. It sounds nothing like it spelt. :L
Okay, I haven't read your story but I know subtle ways to hint that a person has leukemia. I've wrote a worldwarthreenuclearbombingraditionsopeo... story, so I know this stuff from my own research for that story, sorta.
So, nose bleeds is the first one. Leukemia is cancer of the blood and bone marrow. Usually, there isn't a tumor or growth, but it depends, I guess. The next is fainting. If the blood isn't working properly due to a mutation in cells, then oxygen and carbon dioxide isn't going to get to the brain on time, depending on the circumstances, so this may induce fainting. Of course, the third is dizziness as this is associated with fainting. The next is bone pain. This feels like an ache deep in your bones. Again, this is probably (I'm guessing here) associated with the mutations taking place within the bone marrow. Forth is blue lips and fingertips. This is to do with the oxygen thing again. When oxygen is used up in the blood, the blood turns from red to blue. This blood is rich in carbon dioxide and low in oxygen. This happens in leukemia patients because of mutations in the blood (I think).
I know that these things do happen with leukemia patients but my reasons behind these things may not be completely true because I'm picking these from my brain. I'd do your own research on these symptoms before putting them in your story.
Fifth, excessive bruising along the vertebral column... or spine, whichever you want to call it. Blood isn't working -> bruises. That's a very vague reason, I know. Forgive me, I'm tired (:
Sixth, petechiae. These are tiny purplish-red spots on the skin caused by tiny blood capillaries bursting.
Seventh, flu-like symptoms; headaches, chills, fever, night sweats, cough, runny nose, the lot.
Eighth, weight loss. Something to do with an enlarged liver or spleen.
Ninth, cuts won't scab quickly. This is due to a lack of functioning platelets.
Tenth, anemia. Anemia is where there isn't enough iron in the blood.
There is ten subtle ways to hint at leukemia. Before you even think about writing about a cancer, you need to do a lot of research. I spent days on research for my story involving cancer. That was back in 2009 I did my research, hence why my reasoning is so bad. Sadly, the story wandered into a brick wall and was never finished.
The reason as to why you need to know just about everything from platelets to leukocytes to erythrocytes is because if you get one thing wrong, people will know. People will go think that the author was too lazy to go do their research beforehand. Cancer isn't something you can make up. There are people out there who will criticize you for getting something wrong.
Also, you need to decide what TYPE of leukemia your girl has. There are loads. Some of more common than others. Make sure the type you choose is age appropriate as well. Not many say 6 year old kids develop a colon cancer common in obese people. For a example, a 16 year old girl would not be likely to develop cancer all of a sudden, instead she would have been suffering from say Acute Lymphoblastic Leukemia since she was a child. This type of leukemia is something to do with DNA.
All I can say is, DO YOUR RESEARCH. If this is apart of research, I salute you. Research everything I've mentioned. Wikipedia is priceless. Just make sure you cross reference your findings with more reliable websites as well.