It depends on the kids. I have had young babysitters watch my three littles with NO issues, and there are others I won't even ask because they aren't the capable, confident type. So it depends on your niece. But there's nothing particularly challenging about a few hours' day trip that would bar any high school babysitter unless they were particularly incompetent, in my mind.
Then again, I'm a pretty relaxed mommy. It all rests on what you are comfortable with. My kids are easy, my babysitters (younger and older) are experienced with young, closely spaced children, and the timeframe is manipulated so that movies, snacks, and a nap are really all that is required.
Good luck with this, either way! If you're not comfortable skip the trip, but nothing threw up a red flag to me
