Scott, in The Beast Within: The Making of Alien, describes the scene as being accomplished with "a very small person underneath" the Ash model. Presumably this awas small professional adult puppeteer rather than an ordinary-sized child.
"Then they come in and knock his head off, and you go from Ash to a very small person underneath with, and you watch it carefully, with dodgy arms.you go from Ash to a very small person underneath with, and you watch it carefully, with dodgy arms. That's the dodgy moment, but it's taken up by the shock, the slack is taken up by "Jeez, it's a robot" and then getting bashed and bashed by Yaphet [Kotto]"
At least part of the scene was filmed with Ian Holm as the decapitated Ash.
Ian Holm: There is a scene in the film when Ripley discovers that Ash is the robot working for the corporation which as instructed him to bring the alien back to Earth. She announces that she will have to oppose him and he goes berserk but in a rather cool and even calculating way. He hits her and then (in what I took to be a kind of rape) tries to kill her by driving a rolled up magazine down her throat. At that point, other crew members intervene and eventually managed to dislodge my head with, I think, the aid of a metal bar. I remember it as being a very uncomfortable experience. My real head was tucked out of sight somewhere beneath the prosthetic one, and keeping it that way involved all sorts of awkward bending and wrenching. The inside of the phoney head was packed with a mixture of organic and synthetic materials - plastic tubing mixing up with spaghetti and onion rings - to help confirm Ash's humanity