I feel like one is not enough. The group of them are laughing together, something has just happened that froze them in a moment of merriment, it does indeed feel whimsical. Jolly even. One by itself is just creepily laughing because it's about to eat your soul.
I’m picturing a Halloween setup lacking any apparent decorations; a plain yard, nondescript tree, holiday agnostic yard decorations, and a peculiar patternless spotlight shining from behind the rock garden that you have to turn away from to avoid being blinded. Trick or treaters trot their way up the steps and instinctively shield their eyes from the luminous assault to continue the holiday ritual and claim their saccharine bounty. Bags slightly fuller, they turn around to notice the tree shielding them from the light, revealing the rock garden to be a lustrous hellscape of crimson, bones, and grinning mouths. Unimpressed, they continue trick or treating.
Probably a piece, those false teeth probably cost a buck or two a piece and then factoring in the time and effort to either chisel out or melt a cavity into the rock for the teeth to go into