The context of the statement in the question is the important thing. In simple terms, current takes the path of least resistance (actually current is inversely proportional to resistance, so a less resistive path will draw more current for a given potential). The statement in question was made in the context of an ungrounded (i.e. no earth connection, totally floating) AC power system. As the article explains, you an loosely think of your feet as being at Earth potential. You can model a child's finger as a resistor. The potential difference between the two sides of the child's finger is much much less than the potential difference between either side of the child's finger and the Earth potential, so current will simply flow through the child's finger rather than its heart.
This is NOT the case in modern (or even not so modern) wiring, as is explained further in the article, so please don't take this statement out of context, as you can easily DIE these days by sticking your finger in a light bulb socket that is not GFI protected. The more likely hazards are better protected though, it's a trade-off. Read the whole article.