Metro Parks has a few ziplines for kids:
Blendon Woods Metro Park: There is a zipline that passes over a ravine in the Natural Play area
Glacier Ridge Metro Park: There is a zipline in the Natural Play area
Highbanks Metro Park: There is a zipline that is in the Big Meadows Picnic area
Quarry Trails Metro Park: There is a zipline in the south basin of the park near Turtle Cove