A.D.
If you trust this sitter overall and like her, don't let this one incident end the "relationship". Like others said, the "parent's" bedroom might not have been off limits her in house and honestly, a messy room is not a deterrent for a teen or college kid!
I could easily see one of the kids saying, "Let's watch TV in my mom and dad's room...it's more comfortable in there".
If a child I was babysitting for said that, and it seemed normal routine for them, I would likely follow their lead as long as nothing is being disrupted or hurt. Now, if there was a sign on the door saying "Do not enter" or I was told by the parents NOT to go in there...I would absolutely honor their request no matter what the child said.
Just wanted to put a different spin on the issue. ALSO, do the kids dislike this girl? I remember as a child having a babysitter we didn't like...and we came up with all kinds of ways of getting her in trouble and "tattling" on her after we "baited" her (poor girl, now that I think of it! LOL)
Good luck!
(I'm still hesitant to use a sitter other than good friends and family, and my daughter is 19 months old already!)