ظ is the fourth letter that makes /z/ in Urdu (with ز, ذ, ض). All four are pronounced the same, distinguished only in spelling. ظ is rare; it appears mostly in Arabic-loanwords like ظلم, ظہر, ظاہر. The child does not need to memorise rules for which /z/ goes where. Reading widely teaches it. Stroke timings approximate.

Recognising ظ in ظلم and not confusing it with ز in زمین is the eye learning its work. Praise the work, not just the look of the letter.