I am going to assume that we are talking about the tor hidden network. Normal tor usage is relative safe, through https is a major safety net.

Risk and safety is hard to talk about, mostly because its hard to quantify it. I could say that the hacker risk is as risky as visiting a private hosted word press blog, through I suspect more blogs has viruses on them that tor pages has.

As for seeing child porn, so long you don't go looking for it, I suspect the risk is low. Any site that allows users to upload images (like forums) or video without pre-reviewing it could have such images. To be sure, turn of images in your browser when visiting such sites.