Speak Spanish. The best way to teach a child a language is to speak it with them. The child will then begin to learn to think in that language. Have one parent speak Spanish and have the other one speak English to the child. This will help the child develop skills in both languages.
Read age appropriate Spanish children's books to the child. These books will help the child identify words, and build vocabulary. Have the child follow the words along with the adult. When the child can, have them read the book with the adult. The adult can correct pronunciation.
Play language games with the child. There are a number of educational language games and finding the ones that best suit the child is not difficult. Flash cards, workbooks and Spanish board games can help the child develop a working knowledge of both written and oral Spanish.
Watch television. Have the child watch Spanish shows on television. Most television providers offer multiple Spanish speaking stations. Children's shows are usually on during the morning or after school. Turn on the closed captions so that the child will also read the text of what is being said as well.