Furthermore, admissible only matters to a limited extent. It means someone won't go to jail based on this evidence alone. It doesn't mean it doesn't get considered at all. Even then, there's hacks, you could have someone from the company testify to the results of the kit and what the results mean, for example.

Using these kids deprives the government and public prosecutor of many options and places a lot of power with the individual. That weakens the case, but that may be a good trade for people. I'm not sure giving people extra options between keeping silent and going full-on nuclear with zero control and uncertain result (the government route) is such a bad thing.

I can also see why a person with this job (and the conviction that makes one take a job like this) would consider it a terrible thing.