If you're thinking in practical terms, it is probably better to have your kids learn the language of a country that has a significant population *and* has a comparable salary to that of the US. From that standpoint, it's pointless to learn either Spanish or Chinese, since they can hire lower-paid locals with English skills. Think French, German or Italian.
But as Americans, there are significant business opportunities in Latin America and China. I am an entrepreneur and I would want my kids to explore that as well.