Has anybody done this?
Not to my knowledge but I'm sure if you look hard enough you'll find someone who has done pretty much anything/everything.
Would my parents have to declare it even if i keep it under the level?
Yes. Keeping it under the threshold just means that they aren't liable for any tax on the money, they must still declare to remain tax compliant. Lots of info on the rent a room scheme available on AAM and
here.
And would there own income affect this?
Their eligibility for the rent a room scheme?
No. Check out the official details on RaRS for full information on all requirements/conditions.
I'm not sure if thats what this question was in relation too though so feel free to clarify if referring to something else.
Remember if you do go this way to claim the additional Rent Allowance (additional tax credits) available to you as mentioned above.