The English language currently lacks a third-person pronoun indicating either a male or a female human being. Historically, this role was played by the masculine "he", but this usage has gradually fallen from favor. Today, the plural "they" is often used informally in this way, but this is nonstandard and often awkward.
In formal discussions of first language acquisition, the lack of a gender-neutral pronoun presents a special problem, as numerous references must be made to "the child". Phrases like "he or she" get the job done but can be bothersome, especially in sentences with numerous pronouns such as, "He or she hurt himself or herself on his or her bike."
Books on language acquisition tackle the problem in different ways. Some stick to the traditional masculine form, some use constructions like "he or she" and just try to keep them to a minimum (Pinker 1996), some alternate between "he" and "she" randomly (Aitchison 1983) or by chapter (O'Grady 2005), and others make all references to "the child" feminine, using the masculine only for contrast (Tomasello 2003).
I choose a different solution. Following the pattern of the third-person plural, I use the word "ey" to mean "he or she", "em" as the object form ("him or her"), and "eir" as the possessive ("his or her"). This is essentially the system popularized by Michael Spivak (1982) in The Joy of TeX.