The Boxcar Children question

this book is placed in australia and is about a child who was with his parents at a boardwalk and they lost him. he was kidnapped and the kidnapper was sending the mom notes at a coffee shop down the street. the cover looked like a child walking down a boardwalk and it was blue. the title had 'the child' or 'the children' somewhere in the title. if you can think of the name, please, tell me.