Well, the movie origin story was an original creation, drawing some portions from various short stories about Conan written by his creator, Robert E. Howard; the original stories and even the comics did not have the wheel at all.
Is it realistic that Conan would be built up like that from pushing the wheel? Not unless they gave him frequent breaks to eat tons of meat and do various exercises to build up his other muscle groups. If you assume that, as a slave, he was given meager rations, he would have been a rather emaciated young man and wouldn’t likely have survived as long as he did, especially as the others dropped off. The implication in the movie is that the other kids most likely died off, though I suppose a few could have been sold to slaveowners. Why would they take the others instead of Conan? Maybe they saw that Conan would be potentially too strong-willed for them to handle or something. Why weren’t they replaced? Because if they replaced them, Conan wouldn’t have the opportunity to show how strong he is by pushing the wheel himself.
As cuckoorex correctly observed, the story for the 1982 film features a lot of stuff not in Robert E. Howard’s stories, and Conan’s upbringing in slavery is part of this. as a big fan of the Conan stories, I can attest that this is not in Howard’s history. I don’t believe he ever has Conan in any extended period of slavery. In his stories, Conan was a young warrior from the northernm country of Cimmeria who took part in the siege of the northern outopost of Venarium, then went south and had various adventures. Different people have tried to fill in his background in different ways – L. Sprague de Camp, Lin Carter, and that bunch for books published by nome press (then Lancer and Ace books), Roy Thomas and the folks at Marvel ciomics, and most recently Harry Turtledove, in Conan of Venarium. Their visions are different and mutually exclusive, bt none of them pictures Conan as being raised in slavery.
The film did. Although de Camop wrote the novelization, he didn’t have anything else to do with the film, IIRC. According to the IMDB page, the script was by John Milius and Oliver Stone (!! Yes, that Milius, who directed, and that Oliver Stone), but the screen story was by the pretty obscure Edward Summer, who I’ll bet is responsible for this detail.
Forget Conan. The drilling-in-the-ancient-world one I want explained is in Atlantis, the Lost Continent, George Pal’s circa 1960 pseudo-historical epic. It had some great scenes and set pieces, but the movie made no sense at all. At one point, a bunch of slaves sneaks off in the night to surreptitiously drill using a Conan-like Big Wheel. They seem to be drillin for Magma in th hopes that this will destroy Atlantis, but:
1.) How do they know this will work (if this is what they’re doing)?
2.) Howcum the Evil Atlanteans have set up this drill in the first place? They obviously did, because it’s a semipermanent installatio that the slaves obviously didn’t put there themselves.
3.) If the Atlanteans put it there, why do the slaves have to be surreptitious about it?
4.) I think this had something to do with digging up Atlantean power crystals, but why would the Atlanteans endanger themselves?