I had wanted my kids to learn a second language - and had the perfect opportunity since the woman who was taking care of them spoke Fresian. But, she was lazy and it was easier not to teach them, so she didn't.

Hell, at that age, you just have to speak it, and they will learn!
I suppose that the most useful would be the most common foreign language in your area. Immersion is the best way to teach a kid, in my opinion. Get videos in the chosen language (Spanish, German, French and Italian are the easiest ones to find, and I believe in that order). It's a good starting point.
The more exotic the language, though, the more fun they'll probably have with it, it being almost a 'secret' language between them - and it might be more valuable than knowing, say, Spanish. But, then again, knowing Spanish would probably be more
easily useful, no?
Yeah, I'm gonna stop babbling now.