I like to put a few ounces of water in a 2 liter coke bottle (for weight), tie a fishing line to it and cast it parallel to a line of boys ready to shoot that rabbit as he's reeled in past them.
Use an unshot water-weighted bottle to throw in the air and shoot w/flu-flu's.
If hay rolls are still in the pasture, see who can make the long shot on an "elephant" or flight shoot it from LONG distance- closest wins.
One shooter at a time -Hand toss up some clay pigeons from CLOSE distance - 2 tries to hit one.
Borrow some helium. Add to ballons with a string weighted so they just float across the field at head level or so - I like the powder in the balloons idea someone had, too.
Have a few prizes at the end for the...above winners (maybe have the parents do this part and bring something theirs wants but has to win it to get it) - have something for the young-uns regardless.
- flu-flu's, pocket knives (if parents approve), Cheap LED flash lights, sling shots, tools,camp saw, first-aid kit, leather crafts, tabs, etc.
Maybe a hot-dog grill at the end, and a few bales or 3-d targets set up in a row while you supervise and 2 of the parents man the grill