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.

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.