Do you hear your toddler producing /s/ with their tongue out? It’s never too early to model the correct production and try to correct it. Lisps are often very difficult to correct as a child gets older, so our motto is the earlier the better!
This part is too complicated for toddlers, but just so you know as an adult we produce the /s/ sound by putting our tongue tip on the alveolar ridge (bumpy ridge right behind our top teeth). You can show a toddler this by having them look at you or looking at a mirror while you overemphasize the sound to try to show them where their tongue goes. Sometimes we even like to use a tongue depressor or our finger to show them where our bumpy ridge is – for older children we even put a little bit of peanut butter on the spot so they know where their tongue is supposed to touch.
As for the manner in which the sound is produced it is called a fricative, which means it is a “hissing” type sound and air escapes through the teeth causing friction. One thing you can do which works with my toddler is bringing your teeth together or telling them to bite down while producing /s/. This may sound a bit exaggerated, but it makes sure their tongue does not come out.
Many other articulation errors are age-appropriate and can be categorized into phonological processes or patterns, but it is definitely important to keep an eye out and always model correct production. These issues sometimes grow into articulation disorders (which later may manifest themselves into problems with reading and writing) and you want your child to be understood by all listeners and to be able to form friendships easily.