belayer is not really holding the rope and that he is not in line with the device.It is highly unlikely that he will be able to arrest a fall if the kid on the tree lets go.
There are two simple techniques to back someone up who is on flat terrain. The first option is to give the belayer a hip belay. And the second option is to simply run the rope through a second device on the backup belayer.
A Belayer with a Hip Belay Back-Up
Occasionally I work with kids. In such a setting I tend to add yet another piece of redundancy to the system. I employ a backup belayer as well as a knot-tyer. In other words, I have a kid tie backup knots every six or eight feet. This keeps a person occupied who would otherwise be a potential crag management hazard. Admittedly, tying knots in the rope is overkill with adults and even with competent high school students. But when it comes to middle school kids, the more activities they have the better...
If the belay is running through a GriGri or a Cinch, then it might be okay to have a slightly less radical approach to your backup belay. It doesn't take much to arrest a fall in such a device.
If you are not on flat ground and a backup belayer can get below the belayer, it might be acceptable to simply hold the rope for a backup. This is what is refered to as an inline belay backup.
An Inline Belay Backup
Another option that allows you to hold the rope is to create an inline redirect. In other words, the belay rope runs from the belayers device, to a ground anchor and then back to the backup belayer. In such a situation it is super easy for a backup belayer to arrest a fall by holding the rope.
A Backup Belay Running through a Redirect
Backup belays are an important part of the safety net for the beginner climber. If you're new to climbing don't hesitate to ask for a backup. And if you have the opportunity to teach someone how to belay, always always always employ a belay backup. It could save someone's life!
--Jason D. Martin