Is it ok to tell lies sometimes or should you always be honest?

well when people lie it's not good because that can really hurt their feelings and no one likes to see people done or crying but when a kid mostly girl ask's her parent's can she be the president of the united states and they say yes they're lying to her. the point is if you can look someone in the eyes knowing the truth about something and your not telling them than you must be the worst person . And you may ask but what if it's terrible and they would hate me forever? well than you must tell them because your basically saying that every thing in the world is all sweet candy and flowers. so lying or not saying the truth is the worst thing you could possible do.