Does a child play Ash when he is attacked in Alien?

Question

In a YouTube video on Adam Savage's Tested channel, where he meets with the art teacher from North Bergen High School and they are looking at their Ash prop for the Alien play they created, both Adam and the teacher mention that when Ash is decapitated in Alien he was played by a child, due to I would believe the requirement to have someone be the same height as Ian Holm without a head. Adam specifically mentions this can be seen as Ash's arms are fed from the elbow in this scene and not from the shoulder.

The comments can be seen at 23:10 in the following video:

Do we know if a child played Ash at this point or was it someone else?

Answer 1

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 was 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. 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

_{^{Ian Holm: Acting my Life}}

Answer 2

While Valorum's answer is likely to be correct and we maybe unable to confirm completely if a child did play Ash in the specific scene, further research has allowed me to identify that children were used in scenes in Alien.

Specifically the scene were Kane, Dallas and Lambert are in the Space Jockey chamber, the characters where played by children as the set was a smaller than life size scale.

Two of the children were Ridley Scott's children Jake and Luke. This can be confirmed by the two photos below of children in the Kane and Dallas Nostromo space suits.

While we may not be able to confirm that a child played Ash, we do know that other characters were played by uncredited children.