My son started Suzuki violin at 3.5. He was in a group lesson with 6 students to a teacher. Started using a real violin instead of foam (definitely more fun, but buy a cheap one, cos they tend to drop the violin at that age). Parents are also involved in lesson, so I had to buy my own full size violin too.
Our teacher was really fun and engaging.
You may go on this website to find out more about the courses.
http://www.macsmusicschool.com