In my first language, Russian, there is an idiom ‘to tell a lie with blue eye(s)’ (= as a child, sincerely :))
I’d like to know an English equivalent (if any) – a verb (to tell a lie this way) and a noun (a person who easily and normally does that. The word for a ‘blue-eyed liar’, I mean.)
Can’t honestly think of an alternative expression but I can suggest how we describe someone who could be lying but has an innocent expression on their face at the same time - looking as if butter wouldn’t melt in their mouth.