One day, three children went mountain climbing together. They had to walk for a long time to arrive at their destination. As they took a momentary break on the way, the boy in a blue-and-white shirt saw three strange disc-shaped flying objects with flickering lights flying high up in the sky. (The hyphens might be option in this phrase but I'll have to check, but for now I erred on the safe side.) He pointed at them and said in a shrill voice, “Look, everybody!
They areThey're UFOs .!” (See your other thread for a response to the "shrill-voice" phrase. Also, you could change "They're" to "Those are" if you like. "They are" sounds overly formal to me given the context.) They were absorbed in watching the UFOs. Then, the boy quickly took out his camera from his green bag quicklyand wanted to takepictures of the UFOs. (If he tried to take pictures but was unable to, then say "tried to take" and explain the reason for the failure later in the sentence or in a new sentence after this one. If you mean that he began taking pictures, then say "began taking.") In a very short while, the UFOs disappeared into the sky. (Your introductory phrase is correct but for my taste I'd probably say, "Shortly thereafter." That comes down to personal preference though.) The three children were all disappointed.