Being able to trust oneself as a child is a vital building block for a healthy personality. Unfortunately, children will begin to act out in response to the contradiction they are being told.
Also, when parents tell a child that what they know to be true, in fact is not, they cause their child to choose between trusting themselves and trusting their parents. This is not a choice a child can make and remain intact and healthy.
Researchers at MIT have found that children are not gullible, and can in fact sense when parents are lying to them, causing them to distrust the very people who are their caretakers. Children also know when parents are withholding information.