Spanish was my first language. Although I learned English from the beginning as well, I had an accent and was more comfortable in Spanish when I was small. After we moved to the US, though, my parents stopped speaking Spanish at home, and I gradually forgot it. I always knew it was still in there somewhere, though. I began to study it in college and eventually wound up switching from pre-med to Spanish and I've been teaching it for well over a decade now.
My husband also grew up with both languages, Spanish first and then English, and has been a Spanish professor for more than a decade. It was a foregone conclusion that our children would be bilingual.
It started off well. I spoke Spanish almost exclusively to the children until Ariana was around four. Her Spanish was excellent, as was her English. Then she suddenly started showing a strong preference for English, using it all the time and answering us in English. If you are bilingual, you know how much effort it takes to keep a conversation going in two languages. We got lazy and began answering back in English, and in an amazingly short time, our children forgot much of the Spanish they knew.
Recently, we have made a concentrated effort to bring back fluency in Spanish, despite their protests. Here are a few things that seem to be working for us:
Spanish Days: At least a few times a week, I declare a Spanish day. That means that I speak only Spanish to them, and that any TV/music/computer use must be in Spanish also. They grumbled a lot at first, but now go along with it pretty well.
Surround sound: If your children don't already know a language, the amount they will learn from watching movies in a foreign language, etc, is negligible. However, with a background and/or reinforcement at home, they can pick up quite a lot. It isn't just a matter of vocabulary. They are also learning pronunciation, intonation patterns, etc. Listen to Spanish music in the car, watch TV or DVDs in Spanish. Read stories in Spanish aloud.
Travel: Obviously, this one may be more difficult, but if you can spend time in countries, especially for longer trips where they will have the opportunity to play with other kids, they will pick up a lot. They may also acquire motivation. Joel loves Puerto Rico and is determined to learn Spanish so that he can live there some day.
Keep it fun. Sing silly songs in the target language, dance together, cook together. If you are cooking, share traditions and culture, talk about the importance of certain ingredients and their origin. There are lots of happy ways to work things in.
If there is a specific reason why they don't want to speak the language, and you can find out why, you may be able to help. Were they teased or misunderstood by someone? They may not be able to articulate a reason. Often children can't put into words their desire to fit in with those around them.
Don't frustrate them. Even on Spanish days, I will translate into English if they really don't get it (although now Ariana understands virtually everything and translates it for me). If I can tell everyone is getting cranky, we take a break. I want them to develop a life-long love for the language, and don't want to make it into something negative.
Be encouraged. They may know more than you think. Even when I couldn't remember Spanish, I would still have dreams in Spanish, but I would need to tell you about them in English. A few times, when I was very tired, it was like a switch flipped. Once I was writing a letter to a friend who was taking Spanish at school, and Spanish words started coming more easily. I wrote a couple of pages, but the next morning couldn't understand half of it. I thought she would find it funny, so I sent it anyway. She couldn't read it all, but her Spanish teacher told her it was college level Spanish! If they have forgotten what they used to know, remember that it is still there somewhere.
Be patient with everyone, including yourself. Proficiency takes time, even for children. Don't give up. It is never too late, and every little bit helps. Be gentle with any correction. In fact, it is often better not to correct at all other than by modeling unless asked. I experience times in both languages where there is a short circuit between my brain and tongue, and things that I know perfectly well come out wrong. I hear it and wonder how on earth it happened. It is frustrating and embarrassing, but I've learned to just ignore it and go on.
Welcome support from other members of your family or community, even if they speak differently from you. My Puerto Rican mother in law had my two year old proudly showing off her "pinche". It was a lovely little barrette. Being from Mexico, I had a very different interpretation of that word! We used it as an opportunity for all of us to broaden our vocabulary.
Keep challenging yourself. I feel very comfortable with Spanish grammar, but find myself occasionally blanking out on vocab that I should know. Reading in Spanish helps a lot, as does listening to everything I can.
Persistence is the key. Just keep using the language every time you think of it. It is hard, especially if you are surrounded by the dominant language. I know some people have success with rules about one parent using one language and the other using a different one. If that works for you, great! If not, don't be afraid to experiment with what works best for your family.
I have been delighted to see how far the kidlets have come just in the last couple of months. They still aren't speaking it as much as English, but they have gained back much of what was lost, and as their confidence grows, so does their desire to practice. ¡Buena suerte!