Isn't it wonderful when our kids are so specific about what they want??
The only thing that comes to mind is getting a small hula hoop for the base of a coiled snake. You can make a sort of cone with green fabric, using black fabric paints to draw the individual coils and snake skin, gathering each coil in a little more as you work up. Using a baby rattle in the tail would be really cool! Remember the reflective add-ons for safety, maybe for the eyes? Kids love getting their faces painted. Keep with the green and black scales on the face and add some fangs. Good luck to you!