When you define a word, be sure they understand the words in the definition. Delight in discovery. Talk about a better enjoyment of the song because of knowing what the words mean. Keep the fun flowing. Explain figures of speech rather than asking about them.
Young children think concretely and have a lot of trouble with figurative language.
Teach a few words at a time. Sing, teach, and sing some more. Ask for word meanings, but feel comfortable about supplying the answers if needed. Ask them if there are any other words in the song which they don't know. Avoid a test-like atmosphere.