Robert Marzano and Isabel Beck are two influential education researchers who study vocabulary development in children. I won’t be able to do them justice in a short article so you may want to go directly to the source to learn more. Both researchers have books, papers and videos that are easy to find with a simple internet search.
In the book Classroom Instruction that Works (Marzano, Pickering and Pollock, 2001), the authors list five key generalizations about vocabulary instruction: