An article in today's Guardian rather explicitly demonstrates the problem with the publishing industry today. (Note: If anything, the income of US-based writers is worse.) In response to a survey, 117 authors summarized last year's earnings from their books.
Keep in mind, too, that this represents a single-year snapshot, not an average over several years… which is what one would need to know before "quitting the day job." In other words, a children's author—and remember, this is a select group: those who have been commercially published—should probably expect to continue that job waiting tables.
Not very encouraging, is it?