I do find it interesting that you don't have more instruction in pediatrics other than one paper. You don't even have a pediatrics lecture course? Children are NOT simply little adults. Their illnesses and medical needs are much different from the adult population. It's not enough to just study well children. You should have at least a lecture course, if not a clinical, on the topic of pediatrics. I would seriously be addressing this with the Dean of your program. Make sure you check whether or not your school is accredited, since they are lacking this rather vital piece of your education.
Okay, there's my rant.
If you're interested in peds oncology, the first thing I would do is try to get a clinical rotation in oncology. Cancer in children is treated pretty similarly to cancer in adults, so an oncology clinical, even if your patients are adults, will give you great exposure to the field. It will also teach you important information about types of cancers, administering chemotherapy, death and dying, and communication with terminally ill patients.
Second, investigate your local hospitals and children's hospitals. If they have a peds oncology unit or even just a peds unit, try to volunteer or shadow a nurse for a few weeks. (In hospitals without specific oncology units, the children being treated for cancers are kept on the general floor.) If you can't shadow a nurse, try to volunteer with the Child Life department and let them know you have an interest in oncology patients.
Also, see if there are any outpatient oncology treatment centers in your your area. This is where children who are being treated for cancer, but who are well enough to be at home, go to get their chemotherapy, blood work and transfusions. It would also be a great place to shadow.