Everyone lies, give your kid a break, it sounds as though you are being too intrusive in your child life and they believe they have to be perfect. This is one study of many taken Gad Saad Ph.D. as an example:www.psychologytoday.com/blog/homo-consumericus/201111/how-often-do-people-lie-in-their-daily-lives
Participants were asked how many lies they had told in the past twenty-four hours. Here are some of the key findings:
(1) The average number of lies told per day was 1.65. This strikes me as surprisingly low. I have the feeling that many participants were lying about the extent of their lying!
(2) Only 40.1% of the sample reported telling a lie in the past 24 hours.
(3) 22.7% of all lies were told by one percent of the sample, and half of all of the lies were told 5.3% of the sample.
(4) Subsequent to controlling for various demographic variables, no statistically significant sex differences were found in terms of the extent of lying (men = 1.93 lies; women = 1.39 lies). I should mention that evolutionary psychologists would have predicted this null effect, as one should expect that men and women are equally adept at lying albeit they are more or less likely to lie about sex-specific issues of evolutionary import (e.g., in personal ads/online dating, men lie about their social status while women lie about their appearance; cf. Hall et al., 2010(link is external)).
This leads me to one final thought. Men, if ever asked the following question by your significant other (female): "Do these jeans make me look fat?" If the veridical answer is "yes," you may wish to exercise your daily quota of lying in offering a response.
This shows some lying is beneficial to human development, just be thankful you can tell when your child is lying, this could be a life saver for them one day.