Primary

The question came up "What is primary?" I'm not articulate enough to give an answer straight from my brain so I went right to mormon.org This is how the glossary defines Primary:

The organization of the Church for children from ages three to twelve. Beginning at three years of age, children receive two hours of religious instruction each Sunday. One hour is spent with a teacher who provides a lesson for an age-specific group, and a second hour is spent in a combined group of children in instruction and singing.

Before the children enter Primary they are in Nursery.
HTH!