Obviously, they want to be believed, so they carefully watch the person they are telling the lie to in order to see if what they are saying is being accepted or not. Don't think that a child, even of tender years, is incapable of easily reading visual reactions. If a young girl told her mother or father that she had seen Santa Claus, she would probably continue her story or not depending on the reaction she got. The fact is, although innocent, the story is still a fabrication, but positive support of that fabrication usually leads to additional fabrication. Now, a child does not have to have positive reinforcement to expand on fabrication.
Expansion can also result from negative reinforcement.
Again, a fabrication, but again an example that a child is capable of expanding a fabrication whether there is positive reinforcement or not. Unfortunately, most people believe that a child will lie about Santa, the Easter Bunny or whether they did their homework or not, but would never lie about something as serious as being touched or sexually molested. What most adults fail to realize is the fact that most children have no real concept about how serious a false allegation is when they make it or what the consequences may be. The do not look at allegations of abuse in the same light as an adult does. Now, I am speaking about false allegations, not genuine abuse, but when a child does make a false allegation, they are not consumed with the same internal rage that the adult who hears about it is, at least not initially.