From the time my kids were toddlers we always told them if they didn't tell the truth a purple dot showed up on kids foreheads that only their mommies and daddy's could see. Around here it's worked wonders and when mine really aren't lying they tell us to check for the purple dot of if they think a sibling is lying they have us check them for the dot. When mine are lying they try to cover up their foreheads somehow (look away, put on hats/hoods, or twirl their hair right above their foreheads). It's pretty silly, but it was an easy solution for us without over reacting and punishing them for something they weren't quite capable of understanding the problem doing at such a young age.