ص makes the same /s/ sound as س in Urdu. The two letters are not pronounced differently; they are spelled differently because Urdu kept the original Arabic spellings of the words it borrowed. So do not tell your child ص is a 'harder s' or a 'deeper s', because in their mouth it will not be. Tell them instead: certain words are written with ص, by tradition, and صبر, صبح, صدر are three of them. Spelling here is a memory of where the word came from, not a different sound.

ص carries words like صبر and صبح, words your child will hear in family duas and morning greetings their whole life. Tell them: when you write this letter, you are writing the spelling that traveled into Urdu from far away and never gave up its shape.