Your experience is typical of everything that I have seen as well. I have worked hard at teaching my children all three languages, and about their culture as well. I would like to think that they will pass this on to their own children, but if I am honest with myself, I know that they won’t. Chances are that if I want my grandchildren to learn Spanish or Portuguese, I will have to teach them myself.
It's really a fascinating topic. I'll bet there are a number of academic studies on point, although I haven't seen them.
I got interested in the matter after my wife and I made the acquaintance of several ethnic Chinese immigrants from the tiny island nation of Mauritius.
These folks are totally at ease in four languages: English, standard Parisian French, the Hakka dialect of Chinese, and a French-based Creole.
As it turns out, they speak English with us and other American friends, Creole with their Mauritian friends, Chinese with their parents, and standard French at work -- switching back and forth without a second thought.
And of course, their kids adamantly refuse to speak anything but the "Britney Spears version" of English!