Avg stock market return depends on time period selected and type of stock (total U.S., S&P 500,, Global, etc.), but 11.9% is high based on historical average and very optimistic now given that stocks are highly valued. No one can predict future, but most experts think returns in near/ medium term will be lower than historical average given current high valuations.
Also, stock returns have huge variability and could be flat or even decline over short time periods.
This seems to be risky for both you and your parents.