My school has a Junior Primary class, for remediation for students who completed Kindergarten, but aren't ready for 1st Grade yet. It's a small class (8 students!), so I can accomplish a lot with them. This week, they completed heart boxes. The bottom and the lid are a slab cut out with a cookie cutter. For the sides, we rolled a long slab, and traced the edges of a 12-inch ruler to make a long skinny slab the size and shape of the rectangular ruler. The diameter of the heart cutter turned out to be exactly 12 inches!
With this class, the lesson went smoothly with 8 6-year-olds. But, if you were to try it with a larger class of 20 or 30 students, I would recommend teaching it to older students; perhaps 3rd or 4th grade.