At some point during your child's early life, they will lie to you. It is aggravating and frustrating for parents, but inevitably it happens. Our hope as parents is that it does not happen very often. Here is some more information about how to approach this issue.
Why does anyone lie? To get what they want or to avoid punishment. "No, Dad did not give me a cookie today!" (when they have already had their once-a-day treat). "I was not running in the living room!" (when the lamp got knocked over again). "My brother was on the iPad longer than me!" (when they want more iPad time).
Sadly, we are in a current situation where it is hard to point to well known adults that your child may look up to - athletes, politicians, etc. - as good examples of "always tell the truth". This makes it difficult when we all get mixed messages from daily life, including for children. When others see someone not tell the truth then get what they want or avoid punishment, even otherwise well-behaved children (and adults) are more tempted to not to tell the truth.
Certainly, when you know (or are quite certain) that your child is not telling the truth, your best bet as parent is - as always - consistency. By that I mean that you should routinely give your child some negative consequences for lying whenever it happens. We all are more likely to keep repeating an action or behavior if we are not negatively reinforced for it. And we are more likely to stop an action or behavior if we are regularly and consistently negatively reinforced for it.
Those negative consequences may be a time-out, a removal of privileges (no iPad for the day or no cookie for tomorrow), etc. Stick to your guns: if you say it ("no TV today"), follow through with it (they get no TV today). Otherwise, you are accidentally giving your child the idea that they can sometimes "get away with it".
One more note: I think it is important to model good behavior to your children. I truly believe if you model to your child that it is important to tell the truth, they are more likely to tell the truth themselves. I recall a time my one son and I were in a fast-food restaurant. When I handed the cashier money, they gave me back too much money. I said "Here, take the $5 back. Then we are even. You gave me too much cash back." The cashier was happy and I was happy for a small moment to emphasize to my son that telling the truth is important.
So to conclude, do not be too frustrated if your children lie, be consistent with your negative reinforcement, and - as always - call us during routine office hours if you have other concerns.