It depends on the nature of the disability, the level of mildness and the interests and skills of the child in question.

The one that stands out for me, as the younger brother of a friend of mine went this route, is things involving animals or children, as some seem to have higher levels of empathy and assistants in areas that work with children or animals suit them. He got a job at a stables basically shoveling horse shit because he wanted to work and didn't think he'd get anything better... just seemed to have a natural affinity with the horses and ended up being pretty much indispensable to them, just having him there made them much calmer during things like vet check-ups and the like, he had no numerical skills or people skills at all so there was a roof on how far he good go but he did most of the grooming and feeding and was well paid... massive step up from 'shit-shoveler' and he loved the work, just got lucky on finding his niche.

I know it sounds like excessively PC cliched shit, but it really is worth thinking of disabled as differently abled in this case... some have great talent with their hands (Carpenter?) some have incredible memories when taken apart and putting back together (Mechanic?)... It's better to start thinking from the other end, rather than what jobs could they do, what do they like doing? What do they show talent in? Encourage them to try as many things as possible and just see what clicks.

At the end of the day, sad as it is to have to say it, they are less employable, so expect crappy jobs to start with or volunteer work, they are unlikely to walk into something ideal (But to be fair, who does these days!) but it if they are doing something they like and have an aptitude for it will take them somewhere.