Seeing, understanding, and accepting the reality of tragedies are all part of being human and becoming a mature, capable adult. No matter where you go or what you do in life, you will see things happen that are sad, upsetting, and more importantly, unchangeable. It's not a pleasant lesson, but it's a very important one.
I sometimes wish that other people could experience what I did, back when I worked in nursing homes / assisted with hospice care. You get ... very well acquainted with death in many ways and under many circumstances. Not going to lie, some of it shook me up a little, but I learned a lot. I feel better prepared to deal with the inevitable, be it my own, or that of my loved ones. The act of saying goodbye isn't going to hurt less, obviously, but there's something to be said for learning what it is to accept tragedy and to find the good in it.
Also, hearts ... are kind of like broken bones, you know? It's true that if they get really screwed up, it's very hard--rarely, even impossible--to mend them, but sometimes breaking is necessary because when they heal, they're stronger than before. I will add, though, that this was just a game. I know childhood blurs the lines between reality and fantasy, but even so, it shouldn't have any big effect on her. In short: this is normal and she'll be okay.
