The adventures of Vin Fiz - Clive Cussler

Ten-year-old twins Lacey and Casey Nicefolk are fascinated by Sucoh Sucop, the mysterious old man who comes to work on their family's farm.
Whenever Sucop isn't working in the fields he disappears into the barn to secretly work on something that no one is allowed to see. Then suddenly, one day Sucop announces he is leaving but before he goes he leaves Lacey and Casey with a magical gift...

This tiny copper box doesn't look much but if you press its left lever, in a swirl of purple smoke and sparkles, any toy you want will magically become life-size. When the twins decide to try this out with Casey's toy aeroplane, Vin Fiz (named after his favourite soda), they can't even imagine the adventures that lie ahead! Along with their basset hound Floopy, Lacey and Casey find themselves rescuing a town from gold thieves, saving a steamboat on the Mississippi river, stopping a runaway train, catching bandits and stopping a boat from going over Niagara Falls...all with the help of Vin Fiz - who it turns out is no ordinary aeroplane!