The policy at the High Desert Museum does not allow children unattended in the
museum.  However, we have had it happen a few times.  If we cannot
reach the parents at home, the children must check in with the
admissions desk every half hour, or someone goes looking for them.
If they are behaving themselves they are allowed to be alone, if
misbehaving, they are made to sit on a bench until their parents
arrive.  The parents are then told of our policy.
We have had pre-teens left here to do research for school work.  Once
again they need to tell the admissions desk where they are going to
be.  A volunteer or staff member checks up on their progress.
We have not had a problem with children 15 years and younger bringing
in smaller children.  Visitors need a car to get out here, so usually an
adult is with them.  If they are old enough to drive, they should
be responsible enough for their charges.
I hope this helps.
Karla Glanzman
Sales Director
The High Desert Museum
Bend,  Oregon  USA