It sounds extremely obvious and infinitely practical.
How often are you (or your parents) legitimately asked for money via email / internet? I mean, my answer is: never. My parents' answer is: never. My parents-in-law answer is: never.
"You're my daughter? Then tell me the name of your twelth pet centipede."
It should be a simple, pink-elephant-type nonsense question and answer that you rehearse a few times in person and make sure they never share with others. “Q: What’s your favorite leprechaun? A: Rutabaga” or something. Even cognitively impaired folks can, with a few rehearsals over a day or so, retain that easily, I’ve found.
Reminding them to practice it when people reach out pretending to be me takes a bit more rehearsal, but I’ve found that it sticks reliably even then.
Asking for money is common over voice or whatsapp in some countries for things like medical care, funerals, weddings, police "fines", school fees etc.