I agree with what everyone has been saying here but i would like to add one thing: get them interested in the foreign language's culture. When they will become a teenager and you will communicate less and less, the foreign language will get worse. Then, they will move out and only use the foreign language when visiting you, which depending on they relationship you'll have will probably not be daily. Slowly, they will begin to forget the language and will speak with an accent, i've seen it happen with my friends and relatives.

If you get them interested in the culture by watching tv shows, movies, music, books they will be drawn to make friends from the foreign culture to discuss those things. They will discover tv shows or music that they will listen to without you, and maintain the interest when they move out. Basically, you won't be the only contact they have with the language, which means they will use it more and forget it way less.

Anyways, that's just my two cents on this, don't assume that if they speak the language at 10 they still will at 20. I know for a fact my russian is way better than my sister's because i've always been closer to my parents and stuck around when we were watching movies. I grew to appreciate the culture and now a lot of my friends are russian.