One word, “Hydrogen”. Just tell the kiddies to keep it away from open flames...
my guess is that a 60/30,10 helium, hydrogen, nitrogen mix would be ok for balloons, non explosive and relatively cheap.
you only need 30 for the hydrogen because it has a much bigger lifting ability than helium which allows you put in a very cheap 10 percent nitrogen component.
Better still mix it with about 1/3 volume of O2, get a long string, glue black powder or pyrodex around the last 8 inches or so of the string, tie the pyrodex end of the string to the baloon (or tape it), start the string smoldering at the other end, and let it go. You can hear a large one from over a mile away. (The things one does as a college student)