It depends...but probably yes.
What US state?
From their aborted other thread: Wisconsin.
The answer is that police may interrogate minors in your state without parents being present or even informed. It is required now that such interviews be recodred.
As for the DNA, there aren't any special rules for minors to my knowledge. Anybody can voluntarily give up a sample. Those arrested for violent felonies as well as convicted felons and sex offenders must provide a sample. Otherwise, it will take a search warrant.