For this we have a solution.
Most texts on speech and/or language provide charts of studies showing the average age of mastery of phonemes in English for children in the general population.
The text, Born To Talk includes a chart showing the results of three different surveys. It shows that the /s/ sound is not typically mastered by three year old children. Parents need to be aware of this to lessen their anxiety when it occurs.
It would be very unwise to chastise a 3 to 5 year old child for mispronouncing /s/ or to try and cajole him/her to produce a proper /s/ sound.
However, there is no harm in providing sound awareness and discrimination training, which is all receptive to these or any children. This can be done in a paradigm of games or in stories, by simply accenting the sounds when it occurs in the discourse.