My husband is fluent in German. What started as an interest in his family's history turned into a language learning hobby. When our first son came along, we knew we'd aim to teach him German as well. We have a mini library of children's books, DVDs, and music, all in German, for him. Also, my husband incorporates German into our daily activities. Our son recites his German bedtimes stories, sings songs in German, and is learning to count in German. He also uses German words here and there. He can identify both English and German vocabulary words in vocabulary books. He even has played with the children of one of our German friends, and has used his German speaking skills with them. (Mostly he just sang to them in German, or would randomly say German words, such as the word for "sock puppet" in conversation.) He's been doing this for awhile, but we weren't always sure how much of the songs or stories he was understanding. We only translate when he asks us to. (It is recommended that you just use the language without translating, unless they ask you to, when teaching a language to a child.) Well today we got some insight into what he was understanding with these stories.
My husband was reading a story to our son about sheep who are playing together. One sheep is black, the other is white. A group of white sheep come up and see them playing. They tell the white sheep to stop playing with the black one and play with them. She asks the sheep why, and they tell her "Because we don't play with black sheep." Her reply? "Wir spielen nicht mit dummen schaffen." (We don't play with dumb sheep.) When we got to that part, our son said "That's a mean word." So now we know, he is understanding at least the tone and the general idea of the stories and some of the words. He picked up on dummen being mean, even though we always laugh at that part because we think it's brilliant.
So, I encourage parents who want to teach their kids a different language to get out there and do it! Read stories, throw in sentences here and there. Choose some children's music in that language for your kids play list. It pays off, they do get it, and they do think it's fun. My son knows my husband is the German speaker, and he will take his German stories to him to read. On occasion I try to fill in and read one to him, and he'll say "No, Mommy. This is a German story." Sometimes he will even correct my pronunciation. Kids really are sponges. Go for it. You can do eeeet!
It has also been fun for me to learn the language. I understand way more than I can speak, and that helps when we are visiting with German speaking friends, and my husband is speaking to them in German. I have laughed at jokes, and that has surprised my husband (and his friends). I have relayed, in English, entire conversations he's had with German speaking folks. Sometimes my husband will ask me a question in German, and I don't even notice. I just answer him in English. So now, he often will say something to me in German if he doesn't want our son to understand. I guess he will have to change that now that we know our son is understanding more. One day he'll ask, in German, if we can get some ice cream, and our son will answer "Ya!"