I saw some footage about this activity that climbers like to do when they’re not climbing. I searched a little an found a few sites. http://slacklineexpress.com/
There’s a kit you can buy but I made a slackline for the kids out of 1 inch webbing and a couple of ratcheting straps.
It’s tougher than expected but I got my 9 year old son and his friend to stay on it for a couple of seconds.
If anyone here has tried it and has some pointers, please share.