If the country the instance is hosted in has illegalized child porn (japan for example allows some forms of child porn and it's normal to consume those), then the instance will likely take it down fairly quickly. If they don't they might be unfederated by other instances that definitely don't want this.

It's censorship resistant because each instance is an island and yet every island is connected. As long as someone is willing to host your content, people on the fediverse can consume it one way or another.