Documenting dc mL language?

So I'm reading yet another bilingual parenting book and yet again, I'm seeing about "document your child's language." The authors suggest tape and/or video recording. I don't have access to either and I found the descriptions of how to use them rather intrusive and artificial. Also I wonder if any bilingual parents who aren't linguists have actually done this documenting; after all, it's a lot more than writing the child's first word in his/her baby book. I know, at least in part, I could do something like write down the words in a level of Rosetta Stone that he seems to have mastered, but like I commented in my "requiring the mL language" thread, because of some of the parts of German culture that we engage in, he's learning some words that aren't even in the Rosetta Stone levels or not any where near what he's doing (for instance, the word for "to build" in German isn't in the first few units, that I remember, but Finn already knows it because of a Rammstein song) And if I come up with some scheme to track what words he knows, what real use can I put it to? Other than maybe something like "I know you can say 'Tisch' so I expect to hear that rather than table."