Is the word kid slang?

Noun

Some people worry that kid, when used of a child rather than a juvenile goat, is either slang or too colloquial to merit acceptance in standard English.

The fact is that we have been using kid to refer to a child that is human, rather than goatish, for more than three hundred years now. The Oxford English Dictionary notes that although this word was considered “low slang” when it first began being used, it had entered “familiar speech” by the 19th century.

In contemporary English, kid is neither slang nor improper. It is, however, most definitely informal, so those writing professionally or in a formal register might prefer to use child.