Take the boarding pass, if boarding pass is not there then take travel certificate from airline (for this you need to get in touch with airlines, they will provide but they will charge for it. In most of this, certificate is online issue get print out and go to airline office for rubber Stamp (If rubber stamp is not there, immigration won't accept) After getting these take original passport of kids along with 2 copies of these documents and go to immigration office located next to Old airport on 'D' ring road. They will stamped ENTRY stamp immediately without any charges. If you not understand this then first go the same Immigration Office located at 'D' ring road, they will guide you