Finding a reference for Cuchifritín being played by two actors might be hard, you simply just notice it's two different boys when you watch the show.  The reason is probably because small children that age, babies really, can only be asked to "work" for a limited amount of time.  I believe those laws apply in film-making here, too.  Don't worry about the plot summaries, take your time going over them.  I still have to re-write the first episode's summary, too. =]