Do US citizen children need passport to travel within the US?

No US citizen, child or adult, needs to have a passport to travel within the US. There are no border checkpoints between any of the States. However, everyone entering the US from any other country, US citizen or not, must have a passport to enter or re-enter the US. -AY Original answer:

Yes. Everyone needs a passport when going into or leaving a country. If you are in the U.S and are just traveling through the states, then you don't need one.