ض makes the same /z/ sound as ز, ذ, and ظ in modern Urdu pronunciation. The four-letter overlap is an Arabic-loanword spelling matter, not a sound-discrimination one. Tell the child: same sound, different spelling history. The eye learns to recognise which letter belongs in which word; the ear cannot distinguish them. Stroke timings are approximate.

Knowing that ض belongs in ضرور and ز belongs in زندگی is a real skill. Praise the noticing of which letter goes where, not the shape of the letter alone.