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What anime is not safe for children?

It is better to say which are appropriate for children.
People here are listing Berserk and Boku no Pico (which is not safe for adult males of any age), but they are NOT anime for children and that is made pretty clear by their genres. Anime and manga are usually very thoroughly tagged and it is not for nothing.
The genre tags tell people who they are for and what you can expect. This is just a list of genres from Mangafox (genres apply to anime in the same way) There are many, some may be added, some omitted, some may be confusing So safe genres for children are Shoujo and Shounen.
They mark the primary target auditory for the anime/manga. Shoujo - mostly anime targeted at young girls.
It usually features romantic feelings, everything pink and frilly, bunnies, all kinds of fluff.
(Josey is a genre targeting young women - usually with more serous themes, may be a bit depressing).
Do not confuse with Shoujo Ai. Shounen - mostly anime targeted at boys.
They feature fighting (super-hero style) big robots, adventures, and such.
No excessive violence, definitely no excessive nudity.
(Seinen is a genre targeting young men, it is full of nudity and violence).
Do not confuse with Shounen Ai So Berserk is Seinen - full of gore and just pure terror.
Pokemon is Shounen - full of harmless adventures. So one could be relatively safe, if their children watch animes with these two tags, provided that an anime does not have any additional tags that are not welcome. Other unwelcome genres may be: ecchi (erotica) yuri (gay girls often with graphical stuff) (Shoujo Ai genre should have no graphical depictions, only romantic feelings between girls, so may be ok, if you are not homophobic) yaoi (gay guys, often with graphical stuff) (Shounen Ai the same but with romantic feelings between boys) smut (a synonym for graphical sex) mature (can be anything not suited for children: violence, sex, anything) tragedy (some things are very heavy indeed) horror (Japanese horrors can be very disturbing) The rest of the genres are generally fine for children, IF they are not accompanied by the tags listed above. Usually any internet resource that deals with anime and manga have the genres assigned to every item.
If you are watching them somewhere with no genres attached - google any anime database and you’ll have them. EDIT: I forgot to add two genres. hentai is actually anime porn.
If it was released for the internal use it is censored, but even censored… it is not for the people who are faint of heart.
It is… let’s say it can be very inventive. bara genre is in no way safe for any children (even though bara translates as a rose), but I can’t think of any anime in this genre.
There is manga though, which is very explicit and usually depicts very graphical sex between large, often hairy men with little to no plot. Fortunately these genres are usually not found on normal anime/manga sites. And here is one example of an anime perfectly safe for children: Poyopoyo (Poyopoyo Kansatsu Nikki).
It is usually put into Slice of Life genre and while lives can be different, this one truly adorable and can do no harm except give someone cuteness overflow.