Dear parents: never let your kids lie, at any age. It might be cute when they're 3 or 4 or 5, but if you don't gently tell them that "we don't tell lies, we always tell the truth" it will become a habit they will carry into adulthood.
Everybody lies. Sometimes it's good, e.g. in order to avoid someone having a surprise spoiled. Or sometimes it's inconsequential, like telling someone that you're fine because you don't have the mental energy to get into what's bothering you (and you know they won't let it go if you were to just say that). Sometimes it's bad.
Telling your kids "we always tell the truth" is a lie, because you don't. And it has a high likelihood of (as a sibling comment said) making them unprepared for people who do lie, at least while they're still young. And you can bet that the first time they catch you in a lie, it will be that much worse.