I love teaching language to children. "Teaching" might not be the right word. Perhaps it is more accurate to say I love surrounding children with language. Their brains absorb the substance in which they're absorbed...or something like that.
Some children react to a new language by listening. They stop. Their eyes become wide--and they seem to be full of wonder. Some children cooperate in silent recognition that the new language serves the same purpose as their first language. They just "get it" that communication is happening, and they often answer questions correctly, but in their mother tongue. Other children rebel. "No!" they holler as they hide under the table. (Seems like that reaction is most common among children whose language teacher is related by blood--and who is switching out languages on them on the sly, without precedent.) A few children will mimic--parroting back sounds and words, often to the delight of their parents, who are footing the bill for the lessons.
I have enjoyed reading about Ana Lomba's language classes in Princeton, New Jersey, and recommending her CD and book _Play and Learn Spanish_, available at a bookstore, library, or www.analomba.com.
I teach all ages--and have had students in their eighties and others under a year old. I love working with self-motivated students, who are learning for a purpose. When learners are pre-school age, their purpose is not well-defined, but they are driven by sheer curiosity to explore, to test, to try, and to expand their horizons, linguistic and otherwise. If you're a parent, consider starting your child on languages from the start. Google "bilingual baby" to get some ideas. You don't have to be fluent to be part of the child's linguistically diverse environment. Have fun!