Could this also mean, 'Don't tell'?
yes
Paid a ...... was something I heard a lot as a kid
Does this mean "you don't say!" or just a command?
It's a command, wch denotes the imperative form in Welsh.
Peidio is alway followed by 'â'. It should say 'pediwch â dweud'
It's not always followed by â, although it can be. Both forms should be accepted, so report it if it's not.