The /s/ sound is one of the most misarticulated sounds. Sometimes it is omitted so that /sun/ becomes /un/, sometimes the tongue protrudes instead of staying on the alveolar ridge so that /sun/ becomes /thun/. Other times the airflow escapes from the sides of the mouth and /sun/ becomes a slushy /shun/.
To form the sound correctly, the tip of the tongue is on the alveolar ridge (the hump behind the upper front incisors). The sides of the tongue touch the sides of the teeth. The air travels over the center of the tongue. It is sometimes referred to as “the snake sound: s-s-s-s.”
For some children it is easier to start with a "t" sound for placement. Repeat the "t", then move to the "s" like "t-t-t-sssss". Once the child can repeat "t-t-t-sssss" accurately, try words that end with "ts".
Many children have this sound by 5 years of age, but others may not until after the permanent front teeth are grown.
We have free downloads for /s/ practice:
lotto card for /s/ in the initial position
lotto card for /s/ in the medial position
lotto card for /s/ in the final position