I think there is a point to the theory you mention, Man, but it is not set in stone. There are exceptions to the rule; highly motivated and/or highly gifted people. Though I agree that after a certain age, a learning approach works best (doesn't mean you can't still get good results, but you get there in a different way).
Also it does not mean that if you are not perfectly fluent/grammatically correct/whatever by age 6, you cannot ever get there. (come to think of it, I know monolingual 6yos who still struggle with some of the phonetics of their mother tongue, still make basic grammatical mistakes and have a fairly limited vocabulary. Some of them go on being lingustically very limited- but not all).
What it does mean, is that any early linguistic input is good.
So I would say- it may never be too late, but any effort you make now, kanchan, is going to save them far bigger efforts in the long run. Plus you have no guarantee that they will want to make that much bigger effort later in life. Now is the time when you can decide what they do. I would try to speak more Spanish. Do activities that will mean you naturally speak Spanish, like reading them a Spanish story book at night, watch Spanish DVDs together and talk about what you see.
And Broucek, couldn't you encourage your ds to do a couse in Czech?
I have been on several of these threads arguing that you don't have to go OPOL- but have to admit, that if you find you can't keep up a decent level of your own language, then OPOL may well be the way to go.