How in the world did these kids learn to build functioning Go-Carts?
Well, they didn't have videogames or the internet. They had to find something to do.
Well yeah, but it's funny how these kids are at the very least 5-7 years old, and they are smart enough to build Go-Carts that can move..
Well, there are kits you can buy that tell you how to build functioning go-karts. It's really not that out of the realm of possibility.
Why was the movie given a setting update? It seems to be set in the 90s.
A lot of movies are given setting updates. It's to make the characters seem "hip and trendy" and relevant to kids of the latest generation. Kind of like how Alvin and the Chipmunks were given "gangsta" style clothing or how The Smurfs takes place in modern-day New York City.
Also saves money to avoid a period setting. It doesn't sound expensive, and wouldn't have changed the plot much, but setting in in the 1930s would mean making sure every building, vehicle, outfit, and prop has to be accurate. It all adds up rather quickly.