Frankenstein and the Whiz Kid by Vic Crume, 1975.

This book is a novelization of a made-for-tv Disney movie called The Whiz Kid and the Carnival Caper (some editions of this book also use that title).  There was also an earlier movie with the same characters called The Whiz Kid and the Mystery at Riverton.  I haven’t been able to find either of these movies for sale or on YouTube.  Both of the movies are based on a character created for a series of books by Clifford B. Hicks.

The “Whiz Kid” is Alvin Fernald, a boy who is always creating amazing inventions.  He has quite a reputation in his town.  Sometimes, people also call him “The Magnificent Brain.”

One day, his sister, Daphne (called “Daffy”), borrows one of Alvin’s rockets, showing a friend how they work.  However, the rocket ends up falling down a storm drain, and Daffy climbs in to retrieve it.  Alvin comes along as Daffy has trouble getting out and gives her directions to the storm drain’s opening.  Daffy follows the directions and gets out, but while she’s still walking around in the storm drain, she sees a mysterious man with a gun.  Fortunately, he doesn’t spot her, but she wonders what he was doing in the storm drain with a gun.

The place where the storm drain comes out is near a carnival that has come to town.  Alvin, Daffy, and Alvin’s friend Shoie want to go to the carnival, but they need some money.  To get some, Alvin brings out one of his earlier invention, a car-washing machine.  The others are dubious about that invention because it has caused problems before, but Alvin says that he’s fixed it.

Their first prospect for a car wash is the person who has moved into a spooky old house in their neighborhood.  It turns out to be a beautiful young woman named Cathy Martin.  Alvin is eager to impress her, but unfortunately, his invention goes haywire and ends up making a mess that the kids have to clean up (as well as making apologies to other people affected by the chaos).  However, Cathy agrees to go to the carnival with Alvin.

The carnival turns out to be an opportunity for another of Alvin’s inventions when the automaton that they’re using as Frankenstein’s monster in a carnival show breaks down.  Alvin also has a robot that he has built, and he offers the use of it to the man who works on the carnival’s automaton, so the show won’t have to close down.  The man accepts Alvin’s offer, and Daffy volunteers to help with the robot’s costume.

Cathy meets them at the carnival, and to Alvin’s annoyance, suggests that the four of them have fun together, instead of just her and Alvin.  The four of them do have fun, but they stumble onto something strange about Cathy.  They spot a man who Alvin and Shoie met at Cathy’s house.  Cathy said that he was her younger brother, on leave from his base, and that he had to be heading back there soon.  But, Alvin and Shoie wonder what he’s doing at the carnival if he’s supposed to be back and his base.  Then, Daffy recognizes him as the man she saw in the storm drain with a gun!  Who is he really?

The kids decide to spy on Cathy’s house, and they learn that the man, called Ernie, and the magician from the carnival, Moroni, are planning a bank robbery and that Cathy is in on their plans.  It’s a terrible disappointment to Alvin because he liked Cathy, but he thinks that they have a duty to tell Police Chief Moody about their plans.

Chief Moody is somewhat skeptical about what the kids overheard, but he and he deputy stakeout the bank.  When nothing happens, he thinks that the kids raised a false alarm, but it turns out that the robbers’ plan is more complicated than they know.