Friends and neighbors are a good resource for teaching Spanish. Since it is a common language, you may have some acquaintances who speak it. Ask them to use Spanish with your children while they are playing with theirs. Usage could include common play words such as "ball" and "throw." This may be an informal network of learning, but its a powerful tool at your disposal.
Your local library usually has books to teach Spanish to children. Such books will feature pictures with the Spanish descriptions below. Go to your library and ask for assistance in finding them for children.
The Internet is a powerful resource, however, it's hard to discern legitimate websites from "fluff" that could hinder rather than help. Many schools and educational institutions have free online learning for children, and these are legitimate. Professor Emeritus James Becker of the University of Northern Iowa recommends a multitude of legitimate websites that teach Spanish to children.
All the these methods can be combined, so the child is exposed to Spanish throughout the day. For example, you could greet your children each day with "buenos dias" (good morning). Throughout the day, you or your friends could introduce Spanish when the children are playing. Later in the evening, you and the child could surf the Internet, looking over educational websites.