When my wife and I want to communicate without our children understanding, we use Spanish. This is sometimes difficult because I've never actually studied any Spanish, so we are limited in what we can say, and it only continues to work because our children don't know Spanish, but it has gotten us through this far, so we'll keep using it. (The only time it didn't work was once our daughter learned that "helado" means "ice cream.")
However, yesterday I was out with my wife and I wanted to say something to her about the women next to us in the store. I couldn't use Spanish, though, because the one woman was speaking Spanish on the phone. (In fact, what I wanted to say was that I didn't know the origin of the one woman's unusual Spanish accent.) I couldn't use English because the women were speaking English to each other. I've tried before to use German with my wife, figuring it's pretty close to English so maybe she can figure it out, but she just asks, "Are you speaking German to me?", and doesn't even try to understand.
Good thing we both know a little Chinese, right? Herbert and Lou Hoover used Chinese to speak in front of White House staff they didn't want eavesdropping. So I began, "Tā shuō Xībānyáyǔ, kěshì--" but my wife interrupted me to say, "Are you speaking Chinese to me? I can only count to 10, you know?" Since I wasn't trying to say something about a number less than 10, I had to stop.
Aside from private conversations with your spouse, all secondary languages are pointless. You can accomplish the same result with just speaking your primary language slowly and excessively loudly.