For two stimulus payments in 2020, your parents need to file 2020 tax return, and claim the "recovery credits", I suggest using an online tax software. IRS will give you refund, which includes their qualified stimulus payments in 2020.
For the one payment in 2021, and if they didn't file any taxes in 2019 and/or 2020, register here.
https://www.irs.gov/coronavirus/get-my-payment
