As superlarz mentioned, Princess Fantasy Faire in DL is a good place to meet princesses. There is generally three inside (the sign board will often indicate two of the three, and then leave the third to be a surprise). Lately, it seems like Merida and Rapunzel (with Flynn) alternate as the princess outside of the Hall. They also present a comic retelling of the Frozen story at the Royal Theatre at Princess Fantasy Faire. It uses the FP/RT system, but it is not connected to the rest of the DL FP system. Princess Tiana can often be found in New Orleans Square meeting guests (her location can vary and she's not out all of the time). I would also recommend that they see who (if anyone) is meeting guests near the entrance to the park (near the Mickey floral). Not uncommonly a princess is out in that area (although, that location is not consistent and there may not be anyone there at any given moment).
Over in DCA, A&E meet guests in the Animation building. There is also the Frozen Sing-Along (the FP for that one is connected to the rest of the DCA system). Princess Sofia (from Disney Junior) sometimes meets guests near the Playhouse Disney building, but she rotates with other Disney Junior characters, so it's hard to say who will be there when they walk by.