I remember that program from when I was in elementary school, and in fact kindergarten and pre-kindergarten. It wasn't presented as voluntary. It wasn't something aggressive or scary, it was just something that was done, but there was no opt-out; it was simply a normal thing to be fingerprinted, photographed, weighed, measured, and then sent onwards to the playground. We were told, as I was told now, that it was so the police could identify us in case of some form of emergency.
So the police have my fingerprints on file, where I'm vulnerable to the astonishingly remote and improbable chance that someone with fingerprints similar to mine will commit some crime and I'll get matched in a shape-matching database, or the more likely (but still improbable) scenario that the government will, for some reason, use it to track me. Is there any realistic chance that the data will be used against me? Because I am a well-mannered white guy working an office job, no. But that doesn't matter.
I never really had a choice. Sure, it's "voluntary", but I know I was never told that. The parents weren't informed before the program. And as a privacy advocate, I am, in retrospect, uncomfortable with the government having information about me they don't really need (my fingerprints) for situations that are likely to help me. Has a kidnapped person ever been found by their fingerprints?
I guess that's why I don't really support the program: those who participate in it aren't given informed consent, aren't at an age where they're likely to understand privacy issues, and aren't likely to get any benefit out of it other than give the police a slightly easier time tracking them down if they eventually need to get hauled off to juvenile detention.