I didn't think the OP was worried that the Purple family would accuse her, or create a scene. Just that she didn't want to make them feel bad, and she was worried they'd assume she was moving because of them.
But i also think she could just move a little. And if she's moving one row below the Purple family, then it's probably not them.
(Or, before you sit in your new seat, brush it off a little bit, so it sort of looks like your -last- seat was dirty, so you hope this one isn't!)