Shadowing a pediatric oncology nurse in San Antonio/Austin area over the summer

It is unlikely you will be allowed to shadow for a summer, especially in a pediatric setting. The process of approving people to be on the floor is lengthy, expensive and litigious due to HIPAA privacy laws and the need to protect a vulnerable population from everything from possible abusers to the simply curious to the extra infection potentials. Since the hospital would be gaining absolutely nothing from you work-wise but would have the expense and liability of you being there, there is zero incentive to allow you to do this. You can't just volunteer for extra clinicals time and get welcomed in. And even if you could, it would not count as experience.

Look into getting a job as a PCT or CNA while in school.

If you're looking in the Austin area, try Seton Family of Hospitals or St. David's... they have multiple hospital sites. However, I agree with the above post... the best thing to do would be to work as a nurse assistant on a floor similar to what you want to do as a nurse. Not only will you get an idea of what to expect, you will also build your resume and get some on-hands experience.