Students in my online TEFL class have been contributing forum messages about how to inspire very young children to learn a second language. Below is my recent response to a post in that forum.
For very young learners (pre-kindergarten) I think if you just play with them, using fun games and activities, but do it using your own language (in this case English), they will quickly adapt and use it with you.
My first playmates, when I was 3 1/2, were a pair of slightly younger twins who lived next door. We had a lot of fun. My mother always asked me to tell her what they were saying. Some time later she explained that they had made up their own language which the adults could not understand, as twins often do. But why couldn’t the adults understand? I had no trouble. Whatever words they were using were certainly perfectly good – words! And the girls had no trouble understanding me. What words was I using? Theirs, my parents' or both? I don't think it mattered. But somehow when I translated for my mother I must have used the words I intuitively knew she would understand.