In David Wright's recent Downs: The History of a Disability he suggests that the treatment was isolation (institutionalization) due to the concerns of eugenics (mongoloidism). John Langdon Downs, however, advocated for particular forms of institutions for mongoloid children ("idiot asylums"), so this became the experience for some Downs children for many children in the UK until the deinstitutionalization movement in the 1960s and '70s. Concerning the actual "treatment", Wright notes that it was a combination of basic education, psychiatry, and pharmacology (Downs regularly used opium to calm children at the Northern Counties Asylum for Idiots, for example). By the twentieth century, special education had developed as a specialized field and was integrated into the daily life of "idiot asylums". Wright asserts in his book that the UK was one of the few Western states that did not sterilize people with Downs, despite the rhetoric of fear present in the early 20th century.

The piggyback on /u/requinball's answer — "isolation"/institutionalization of the mentally disabled was the general course of "treatment" and "education" in the United States through the first half of the 20th century. The quality and methods of such institutional experiences could vary dramatically be state or even individual locality. Mental health institutions in the United States could be operated by public or private means, and even the public institutions could be very "feudal" (in the sense that they were largely run without oversight) until the era of the 1950s or so. As for "treatment," the major forms for the mentally disabled (as opposed to the mentally ill) was what they called "occupational therapy" — teaching basic skills and menial labor and using these as a way to structure their lives. Not the worst option at the time, far more appealing than what got done to the mentally ill (which at various times included lobotomy to electroshock to "therapeutic" sterilization to eventually a battery of pharmacological drugs). Though there are aspects of some places where "occupational therapy" starts to look a lot like "free labor."

At the height of the US eugenics movement (interwar period until just after WWII), the odds of being sterilized at such an institution could be relatively high depending on which state it was in (California had the highest sterilization rate, and it was most consistent at institutions for the mentally disabled).

If you are interested in reading an account of life at an "Institution for the Feeble-Minded" — which is not the same thing as Down's Syndrome but gives you a character of what occupational therapy is like — Henry Goddard's classic eugenics text, The Kallikak Family (1913) contains some interesting descriptions. It is written by the head of the Vineland, NJ, institution that it describes, so as you can imagine it paints a very rosy picture of the whole endeavor, but as a historical document/primary source it is an interesting insight into some aspects of this form of early 20th century treatment, even if it is probably the most "positive" side of that life that could be put forward.