Kids usually lie because they're afraid of the consequences. So how can you make them more comfortable with telling the truth when they've done something wrong?
First of all, make the consequences felt, but not so drastic or painful that your kids feel lying is the better alternative to being truthful. Remember that God is patient and merciful with us, He doesn't strike us down with a bolt of lightning when we mess up, especially when we're "young" in our walk with Him.
Second (and maybe I should've put this first), the best and most powerful way to combat lying is by developing and maintaining a relationship with your child where they feel safe and loved. That isn't something you can work on here and there, it's something you need to do 24/7, all the time. The more your child loves and trusts you, the less incentive there is to lie to you, the more obedient they'll try to be, and the more uncomfortable their little conscience will be if they try to lie.
Lying is based on fear, the Bible says love casts out fear; the same dynamic between us and God applies to us and our children. If a Christian sins and feels afraid to confess it because they're terrified of God smiting them with some terrible punishment, there's something extremely lacking in their relationship with Jesus. Guilt and conviction should drive us TO confess, knowing that forgiveness and grace are just a prayer away. Try to nurture that kind of trust and bond with your child!
Sorry for getting so lengthy, I really didn't mean to, lol. FTR I'm a single mom to a 9 year old boy, and because of the above beliefs and practices, we do not have an issue with lying.