Backyard zipline setup—is it insane?

I've been looking at those backyard zipline kits (Slackline Industries or similar) and my kids are BEGGING for one. We've got the property for it—about 70 feet of clearance between two trees. Kids are 9 and 12, pretty coordinated, not reckless.

Wife thinks it's a lawsuit waiting to happen. I think it's fine supervised play. What's the general consensus here? Anyone actually have one installed? Did your homeowner's insurance say anything about it?