Since it's Mum and Dad's carpet I'd specifically wait till they return before doing anything.
You'll get a far better answer from them as to what their expectations are and you save yourself a load of grief and get the chance to be a hero if they instruct you to make an attempt (once you have fully appraised them of the pro's and con's).
I've always avoided like the plague jobs where, for example, builders want you in to get them out of the "do-do" before the home-owner gets back. Builders most certainly do not like paying for a clean if they end up having to but a new carpet in the end.