The book I am trying to identify was written over 60 years ago, and it features a group of children ages five or so through 14, or thereabouts.  They have been sent to the country because of the Second World War, and they become convinced there is lost treasure in the vicinity.  The only scene I remember takes place in church.  The younger kids are shocked and scandalized during the service as the collection basket enters their pew.  The eldest, instead of putting a coin in, pulls one out!  This is their first real clue.  The coin turns out to be a genuine gold doubloon.  The eldest boy may have been named Emil.  Do you know which book this is?