Asking because I've been told this is the stupidest idea ever, the hot lead would set it on fire in a heartbeat.

Mostly for peace of mind, want to put a backstop in the back pasture to start the Gkids shooting .22LR and I'm thinking a wall of 25-30 small square hay bales would be pretty near perfect. Looking straight back from the house there aren't any buildings or anything until you get to the next section line (1 mile); there are houses left & right but the kids'll be shooting from a line well behind them.

If that's as stupid an idea as I've been told, aside from spending hundreds on a dozer guy to plow me a "range" back there, what better options are available?

IT shouldn't catch on fire unless your shoot tracer rounds. Just make sure its fenced off from livestock. Shoot into a natural hillside or back of a damn. Stay away from riparian areas if you plan on shooting there for a long time.

I don't think it will catch fire, but I would probably find something other than hay bales too.

Why not put a mound of dirt from somewhere else on the property?

I have a buddy who stacked big cedar logs, two deep between some braces.

The hay bales do work, but not the best, for reasons already stated. A few rail road ties work good, or dig a hole and use the dirt for a small berm.

Hay bales would work ok until it came time for disposal, they will fall apart eventually just from age, and you can't feed it 'cause it's full of lead. If at all possible, I'd have a couple 6-yard dumps of fill dirt dropped off. If the driver has room and is worth his salt, and knows what you're planning, you won't even need a tractor. They can loosen the chains on the gate and do a pretty good job of driving out from under it. There's your berm. Now, if at all possible, if you could raise the shooting platform even just a couple steps, so they're shooting somewhat down and into the base of the berm. Perfect. What range are we talking about here? I personally wouldn't use this with kids past about 25-30 yards. If they miss high it's going into the berm. Longer than that, the bbl coud be level or even elevated and that little bullet can fly a looong way.

you can get a sheet of 1/4" AR500 to use backstop

that's what I did on my place

have 8x5, 8x4 1/2" AR500 ..... i can shoot 338 Lapua and not worry about
bullets misbehaving and landing on neighbors