How to explain crucifixion to a child?

I think you need to start with the original context, which was that this was a punishment for criminals. It was meant to be a very painful and public death, so that other people would see it and be afraid and not break the law. I don't know if she understands death yet, so you might have to spend a bit of time there first.

Then you can explain that Jesus was someone who was not a criminal, but he said things that angered the authorities so much that they decided they had to get rid of him. So they had him crucified. You might be able to tie this to current events where people who anger those in power get harassed or imprisoned; this kind of thing has been going on for thousands of years. Its not right, but its very hard to stop.

Finally you can explain how Christians view this whole business and why they see the crucifixion of this particular itinerant preacher as being worthy of great paintings.

Take it one step at a time. You probably won't be able to get through the whole thing in one lecture. Be guided by her questions and don't worry about skipping over details for now.