What should you expect?

-A couple days to a few weeks of court.

No... there's a high probability this will be plead out. Even if it isn't, it's not a complex trial issue requiring more than a day or two. Unless the prosecutor needs your testimonial evidence (and he probably doesn't considering the nature of their apprehension).

What could happen to the kids?

-Jail time

Juvenile hall if the age estimates are correct. Depends how the prosecutor wants to try him. Prosecutor can petition to have him tried as an adult, and that improves his plea bargain positioning (more likely to get the kind of plea he wants). The prosecutor will probably use this to secure a guilty plea and testimony against the passenger, because the truth is... If the kid is actually on probation he'll have to serve out the full time of his previous sentence, which means he is in Juvy until his 18th birthday at minimum. Sentencing decisions vary depending on crowding issues, so he may not serve a day over 18. Prosecutor knows this, kid doesn't...

What will you be expected to do?

-Show evidence of what is stolen, show up to court, make your case.

None of this. At this point, you probably won't have to do anything. The case will be plead out.