If you are not a Spanish speaker, and you do not have a close friend or relative who can offer to teach, you may need to look elsewhere. Consider placing an ad in a local newspaper or on the Internet, looking for a Spanish teacher or native speaker. Instead of payment, consider teaching your new teacher English in exchange for her helping your children with Spanish.
Even if you have no knowledge of or a very basic level of Spanish, you can find the resources that can help you teach beginner's Spanish. There are many excellent free resources online which should help you plan your lessons. Alternatively, try to find a teaching book for the appropriate level of Spanish. You can teach your child at home, or even get a group together.
Whether you find someone to help you, or plan to teach yourself, you will need to be organized to ensure the best chance of success for your child. First, assess which level of Spanish your child has, and plan your lessons around this choice. Plan lessons that consolidate previously taught material while moving on to new material shortly. Adjust lesson expectations to your child's progress.
After an enthusiastic start, it is easy to let both yours and your child's attention wane. Keep a planned diary of lessons and expected progress so that, when your child gets stuck or starts to find things difficult, the lessons don't become neglected. Change topics when things get stale -- revisit difficult topics with a different focus at a later point if it is becoming a sticking point. Persevere with varied lessons and topics to ensure steady progress.