I'm guessing your parents have roughly 60% stock/40% bonds, since their allocation is appropriate and that's a very standard retirement portfolio. Do you have any small accounts where you could imitate your parent's allocations, but have your overall allocation be appropriate for you?
For example, if you allocate 4% bonds / 6% stocks in one account, that could match your parent's allocations. You start it at the end of this year or early 2020. And then you show them your percentage gains, and ask to see if they're getting good results like you are seeing. Would that be a fair way to show them?
Notice I'm not talking about percentages listed in index funds. I'm saying you take real money, with a real return, and show your parents the account statement. I suspect they will believe your personal account statements, so it can get them thinking or talking about it.