This site is hosts adult material that can be viewed as offensive by some people. The site is safe, but you must be at the legal age to view this site (although there is some non-adult material on said site).
Exhentai is a site for doujins which are basically unofficial Japanese comics of anime series or films. Often times, these doujins are erotic or sexual in nature.
This site is made such that it is not easily accessible without certain Firefox or Chrome extensions (impossible to access on IE AFAIK).
So yeah, it's a safe site, but not appropriate for children.