Alien has become a pivotal film in science fiction for a lot of different reasons, but like many films of it's kind it was the inventiveness of the visuals to help create a unique experience. Even little things helped create the awe-inspiring, now iconic, visuals. One involved simply giving off the scale of the primary setting of the story, the Nostromo. Even though a 58-foot (18 m) landing leg was constructed to give a sense of the ship's size for exterior scenes, director Ridley Scott was not convinced that it looked large enough. So, he had his two young sons and the son of Derek Vanlint (the film's cinematographer) stand in for the regular actors, wearing smaller space suits to make the set pieces seem larger which was also the technique they used for the scene in which the crew members encounter the dead alien creature in the derelict spacecraft. Here is a picture even of Scott's kids in the suits during production:
However, and this happened with the spacesuits for the grown actors, but the children nearly collapsed due to the heat of the suits. It was because of this that oxygen systems were eventually added to help the actors breathe. Stay tuned!