For a funny smile you could add a "cavity" by putting a bit of caramel or chocolate syrup on one marshmallow.
Add a chocolate chip or raisin for a "rotten tooth". This would allow for a great conversation starter of what is healthy in this snack, what is not so great and why.
Use whatever butter you need and or prefer: for nut allergies you may be able to use almond (which is actually considered a fruit see) butter or sun (sunflower seed) butter. Natural, homemade, almond, and cashew butters could all be used.