...I'm thinking of buying a bit more, though, so I'm wondering if any of you have had any wines from 2004 from anywhere in the world that you are convinced will age indefinitely (or 25+ years at least).
The general lack of interest in Californian wines here has resulted in a serious gap in the recommendations: Petite Sirah! Petite Sirah, when made from old vines with the intent to be age-worthy, ages at a glacial pace.
IMHO the goal of laying down wines for your kids is not so much that they blossom into perfect examples at age 21 or 30 (it's hard to predict optimum age for drinking, even harder to predict what your kids' taste in wine will be). No, the key factor is the odds of the wine surviving that long in a form that most wine drinkers will still find pleasant. Petite Sirah does this really well, even if it doesn't develop the complexity and refinement of top Barolos, Bordeaux, Hermitage, etc. The best bets that come to mind are:
--Ridge York Creek
--Stags Leap Vineyards (not Stags Leap Wine Cellars)
--Ballentine Fig Tree vineyard
The J. Lohr Tower Road vineyard and the Preston Dry Creek bottlings seemed like good bets lately, but I haven't had any experience aging them. I have had Ridge and Freemark Abbey York Creek Petite Sirahs from the 70s, and Stags Leap Vineyard examples from the 80s, that seemed just middle-aged in their development.
Barolo is a good idea generally in terms of ageability, but the problem is that old Barolo is something of an acquired taste, and you have to be something of a wine geek to appreciate them. Vintage Port, Sauternes and the sweetest Vouvrays are a safer bet in terms of satisfying both the casual or aficionado's palate. But I don't really know how good the 2004 vintage was for long term aging in any of those regions.