The compulsive liars are Julia and Jack.
This can be reasoned by trying to prove a statement that says that one person is certainly lying or certainly telling the truth and checking it.
According to the text, Jack is only a liar. This is stated by Joey when he says that "Jack lies, as does Jane or Julia". Jack says that "If you round up Jane, Joey and John, you will have at least one liar". This means that none of them are liars because Jack, the one saying this, is a liar. Thus, these three (Jane, Joey and John) are the ones that can only say the truth. The text also mentions that two kids are liars. That leaves Julia as the other liar.