Mongolian and Chinese visas on the road

My son and friend (just turned 19)are currently traveling from U.K to china via Russia and Mongolia on the trans Siberian train. They are currently in russia and only have Russian visas in their U.K. Passports. They also have Australian passports. They planned on getting Chinese visas in Ulan Bator but are not sure they can get Mongolian entry permit while traveling to even get there to get their chinese entry permits. They thought they didn't need a transit entry permit to go through Mongolia. It all sounds very vague on line about what is required. Does anyone have advice on what they should do. Surely it can't be that hard?!?