Are you in a public school or another setting? If you are in the schools, I question whether there is an adverse educational impact of a lisp that is "normal" acoustically. Are you able to demonstrate some sort of adverse impact? In addition, an interdental lisp is still considered developmental at age 6 in my district. If you are seeing the child in private practice, then I guess it depends more on whether insurance (or the parents) will pay for your services. Not sure about the ethics in this situation.