Not a peds nurse but I am a heme-onc/BMT nurse, so I get what you're saying. The transplants and chemo themselves aren't usually that bad, but the after effects of chemo and the GVHD is absolutely awful. We've had some poor patients who are in their 20s die due complications.
The hard thing in this field is that we see them suffer and sometimes die, but we don't see all the ones who are flourishing and thriving because of the care and treatments we gave. We give them their last chemo or their transplant, and if everything goes well, we don't see them again. It's when they start getting complications or go into relapse that we see them again.
It's hard to remember all the good when the bad is so hard, but it is out there. Plus, every year research and medicine in cancer is improving.