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.
"I have no idea what WW-III will be fought with, but WW-IV will be fought with sticks and stones."