Most of the times, people learn more effectively by example so I'd suggest you to ask them to give you a mid-sized loan for let's say half or one year. Buy BTC with the money, transfer it to a separate non-custodial wallets for each family member and set an alarm to remind you for loan maturity. When the time comes - gather them to a family meeting and show them the transaction history (preferably using Electrum wallet because it has a nice history page that shows the historical price of each UTXO at the time it came which will help you prove them how much they bought initially and compare it with the current price) and boom 💥 you achieved both:  

  P.S. The bonus you're getting with this method is that they will be forced to also learn how to recover, maintain and use Bitcoin wallet so that they can get their loan back 🤣