Kids are language machines. I don't have any real difficulty getting the basics of a language, but my abilities with Dutch is childish compared with, well, a child's. Apparently there was a time when I didn't speak English, and I damn near flunked grammar in grade school (the difference between a subject and an object confused the bejesus out of me until college). Now, I copyedit stuff.
Given that kids are language machines, then, we're not going to worry overly much about teaching the Little It English and Dutch, other than to teach It that Mommy speaks one language and Papa speaks another. My mother was raised this way--and up until their very last, she spoke one dialect to one and one dialect to the other. Language is part of someone's identity, almost more so than their personalities--people can change, but if you've always spoken Dutch to one person and English to another, then it's really really hard to switch it around.
My own experience in being raised quasi-bilingual (I infinitely prefer English over Chinese) is that formal lessons suck, especially if they take place on Saturdays. If I had to do Chinese school over again, I'd have arranged it to happen after school a few times a week, rather than on (*groan groan groan*) Saturday mornings --I don't know if they've changed it since I stopped going, but it sort of makes more sense, from a psychological point of view: i.e., your week is ruined by school anyway, so a couple extra hours of school isn't going to ruin the week any more. And in any event, nothing sucks worse than writing lines (Chinese writing is typically taught by filling in columns of 12-14 squares with characters) on Friday night. None of which enhanced my proclivity towards learning Chinese. I speak it with my mother, but only because she doesn't always understand English.
Dutch schools typically begin teaching English at a very early age--the typical "Hi, how are you" and counting from one to ten and all that stuff. But it's a kind of forced bilingualism--the kind of Spanish you might learn to speak for a trip to Mexico--rather than a true ability in both languages, and it shows if you try to speak English in the smaller towns and cities outside of the Randstad. I'd rather have a child go to school having never heard a word of Dutch in his life, than to go to school learning appalling habits from me.
Still, I've had one or two conversations with other people, who are surprised and not a little worried that I'm relatively blasè about teaching the Little It Dutch. It doesn't matter, to them, that the science and my personal experiences back me up. It's strange, if you think about it--all the care and attention that goes into prepping children for school, all the reading and "enhancing" activities that you're implored to do, and yet, when it comes to actually letting a child work out that Mommy speaks English and Papa speaks Dutch, they're all terrified that he won't be able to.