The app allows people around the world to send free messages to each other almost instantaneously.
The girls began using the app to talk to a number of people who claimed to be men and who urged the girls to send them naked photographs of themselves.
The men sent explicit messages and indecent images of male body parts and repeated their requests for the girls to return the favour despite the children telling them their ages.
Earlier this year Sussex Police issued a warning after reports of similar incidents involving the social messaging system Kik.
Leanne McSorley, from Worthing response investigation team, said: “All of these men were able to hide behind screen names to preserve their anonymity and prevent the children knowing who they were talking to.
“The two girls are both shocked and very upset about what happened. We know about their case because their parents found out and contacted us but there could be other young girls who are being targeted but who have kept it to themselves.
“One of the men also tried to contact the girls through a computer using Skype but they refused to talk to him.
“It is unclear how old those who have been approaching the children really are or whether they have asked for the images for their own entertainment or to pass among others.
“We are concerned they may pose a risk to them.
“I would urge parents to speak to their children to warn them about the dangers of making friends online, sending indecent images of themselves and in particular agreeing to meet people who they have only met online.”
Anyone who has any information about adults contacting children online inappropriately should call 101, email [email protected] or call the independent charity Crimestoppers anonymously on 0800 555111.