If I were you, I would start off with flashcards or some kind of language game. Kids aren't just going to sit down and go by the "book" because with them, they just can't sit in one place. Dedicate at least fifteen minutes each day for your child to learn a language. As they get older, you can extend their learning time. After, it is a good idea to enroll them in either Spanish, German, or whatever language you want them to learn. Classes will be good for them and can really help.