I do heard that some have been accepted the disabled child but some not. My uncle who has a disabled child actually being accepted but with condition is that all the medical cost shall be beared by themselves instead medicare. Require to sign a health undertaking. They give up as high tax paid does not give any support to the child. No point of staying there.
However, this very much depends on your child's health and disability condition:-

Dependent
The emotional family ties that may exist between children and parents are not regarded as an indicator of
dependency.
To be considered dependent, the child must be in one of the following situations:
•  be wholly or substantially reliant on the sponsoring parent for financial support to meet their basic
needs of food, shelter and clothing and be able to show that they have relied on this support for a
substantial period (usually 12 months); or
•  be wholly or substantially reliant on the sponsoring parent for financial support because of a disability
that prevents them working to support themself. The child will still need to meet Australia’s health
requirements before being granted a visa (see page 24).
‘Disability’ means that the child has total or partial loss of his or her bodily or mental functions. Acceptable
medical evidence must be provided to show that the child has such a disability and they are unable to
financially support themself because of it.
‘Wholly or substantially’ means totally or to a great degree. The child must show that their reliance on
the sponsoring parent for financial support is greater than their reliance for financial support on any other
person or source.

Depicted from this websites: http://www.immi.gov.au/allforms/booklets/1128.pdf

Do seek advice from migration lawyer / agent for further as we are not the party to advice any further on this.

I'm sorry, they actually applying themselves instead going thru agents. My cousin is a downsyndrome too.
My uncle working in oil & gas and he's in China now. That's why he earns pretty high salary and got fed up with the Oz government on the medical cap to his son. If he pay high tax and got none medical benefit to his son then it's no point for him to migrate.