I'd say be extremely realistic about the number of kids you can handle with a demanding career. Any more than 2 is really pushing it IMHO, unless you're OK with someone else doing a lot for your family or you staying at home yourself after all those years spent training..
First of all, it is four years of undergrad, four years of medical school, three years of internal medicine AND THEN three to four years of hematology/oncology. Please, first get your BS/BA or whatever, THEN apply to medical school, then IF accepted go through rotations with an open mind, THEN based on your performance and USMLE's think about what your options are. Thinking about hematology/oncology now is ridiculous when you have yet to even finish undergrad. Not trying to be harsh but just realistic.