People get very upset when others lie to them, or feel that people aren't being truthful, or with holding information, yet everyone does it or has done it in the past. People lie for all sorts of reasons, knowing we wouldn't like the truthful answer, our past reactions to the truth before, shame, guilt, fear of punishment, being judged, the list is endless.
If you are running into this situation frequently you may find it helpful to examine your reaction to this person and see if you are receptive to the truth even if that truth is not what you want to hear. If you get angry hearing it, then consider a more even or less aggressive response, you may find people being more truthful with you.
Keep in mind that telling the truth can be painful for the other person, the more you can demonstrate, that while you may not like what the truth is, you can can show appreciation for hearing the truth and move forward most people will respond favorably in time. This is a good practice especially with your children.