If a child came to you and talked about their twinkle, foof, fanny, tuppence, flower, mini, bits, noonoo, foofoo, willy etc...you'd know exactly what they were talking about. They don't have to say the word 'vagina' or 'penis' so that others understand.
Yes, but what about different cultures that have different nicknames? What if a person was from a different country or hadn't heard that particular slang before? Or what if a child was from a different country and their parents used common slang from there?
Over here we have names that are also common objects, like: Cookie, biscuit, kitty, etc.. If a child came up and said someone was petting their kitty, it wouldn't register as alarming.
But giving a nickname gives the added problem that it gives the mentality that privates are not to be talked about and are embarrassing to discuss which could also deter a child from talking to an adult.