The frontal lisp is a topic near to my heart because my son struggles with this issue. Only slightly. Actually, it is such a minimal amount that only his hyper-aware Speech Pathologist mommy would even notice.
This is my son’s super adorable little mouth. Can you see his tongue just barely sticking out under his front teeth there? No? I can.
My mother, the dentist, is overly aware of teeth. She used to say, “That movie star certainly earns enough to fix those teeth!” whenever she watched tv. I inherited this skill in mental note taking. Both for teeth and speech.
Developmentally, your child should have a good /s/ sound by age 7-8. Very few public school systems will make a child eligible for /s, z/ services before 7.
The technique that I have had the most success with is sitting in front of a mirror with the child, instructing them to keep their tongue behind their teeth, and say “t-t-t-t”. Do you feel where your tongue is there? Can you see where your tongue is? Behind your top teeth. Now add an “s” so that you say “ttttsss”. You are a steam engine letting off steam. No tongue sticking out of your teeth. Get back tongue! Back!
It is very important that the child not alter their /s/ sound into a “slushy sound”, like a “sh”. If that happens stop work right away. That is far more difficult to correct than a frontal /s/.
Work with your Speech Pathologist to find words that encourage good /s/ practice.