Peds oncology? Fun? Your terminology is a relative turn off to the gravity of your question.
I lay no claim to being a peds oncology nurse; have done peds case management. Here it is as straight as I know how to give it with my limited background.
You are seeing children and families when they are experiencing a life changing experience. They are learning that their is no guarantee that they will see their child graduate...from kindergarten, eighth grade or high school. They are coping with uncertainty, seeing their child in pain, death anxiety etc. Some of them cope with all of that with amazing grace and class; others do not. The kids themselves are learning life lessons many 30 year olds haven't faced.
The words that I might associate are gravity, intensity, need for high technical skills, love of children, love of taking care of and interacting with families, tolerance, great communication skills, willingness to feel your own pain and sadness, ability to take care of yourself even on the hard days. there will be fun and laughter and triumph but if you aren't ready for the other aspects, you might just try regular peds and see if you have some of those other skills. Pediatrics nurses are graduates of regular schools of nursing who get hired onto pediatrics units. Nurses who are into peds have some of those features I have already listed.
Good luck in your decision making.