From the Wall Street Journal:
Under the new law, there are several tests to qualify as a child. The child must be your son, daughter, stepchild, eligible foster child, brother, sister, half brother, half sister, stepbrother, stepsister or a descendant of any of them. The child must be under age 19 at the end of the year, or under 24 at the end of the year and a full-time student -- or any age if permanently and totally disabled. The child must have lived with you more than half the year (although there are some exceptions, such as children who were born or died during the year). The child must not have provided more than half of his or her own support for the year. For more details, see Chapter Three of IRS Publication 17 (www.irs.gov).What, you thought it should be simpler than that?
Congress was actually trying to streamline the Code, but we're talking about Congress, after all. Previously there were at least five different tests for what a child is, one for each of the different tax breaks tied to children. So, this is an improvement, right?