When my son was 2 and a half or so he asked me to teach him Spanish. And we did. For about three years we worked on Spanish. It was great fun. I landscaped our house during this time and many of the workers that we hired spoke at least one form of Spanish or another. My son's first word in Spanish was "pala."
At about three, he was in his stroller one day when he leaned back and said "I'm from Mexico." Cool.
And then we went and tossed the poor kid into a German immersion program, losing all of his Spanish!
My child is now in first grade at the local German immersion school. Here are my notes and resources for learning a foreign language. For now, it is very German-intensive. I hope that we can get back to Spanish, though!