Hi,
As the beneficiaries, your parents would be expected to submit the form, P53Z(DB).
You would only be in a position to submit this form on their behalf, if you held a power of attorney for them or held a similar position to act on their behalf. If you do not want to, or cannot submit it online, you can download the form here:
Pension death benefit lump sum: repayment claim
The address to send the completed form to is:
H.M. Revenue and Customs,
Pay As You Earn,
BX9 1AS
Thank you.