Hi. I'm a special education teacher as well (only my certification is in blindness and visual impairment), and I sometimes wondered the same thing. However, I'd like to think that in our day and age parents won't exclude a child from succession just because he/she has special needs. Nowadays monarchs are not involved with politics, but are ceremonial heads of state (at least in Europe), so I don't see why a person with Down Syndrome, Autism, cerebral palsy, visual impairment, etc. won't be able to perform the duties, with assistance when and if necessary.
On the subject of trisomy 21
(down syndrome) I think there must be certain considerations that would need to take place if an heir were to be born with the said chromosomal abnormality.
Fine and gross motor skill delay can interfere with cognitive development and congenital heart disease has risen significantly in relation to new borns
(at around 50%).
Also fertility rates amongst people with down syndrom is significantly low with many males being unable to father children and women displaying low numbers for successful conception rates. As Europe's monarchies exist with a fundamental aim to be inherited, pressumably, by the child/children of the monarch this of course makes the likeliness of it very difficult and would place a great deal of stress upon that person and indeed the wider family.
The possiblility for an early onset of Alzheimers is also believed to be increased for people with trisomy 21. This occuring at an earlier stage of life than as is usually seen with most other people. Also there is an increased risk of developing epilepsy throughout childhood and adulthood.
Furthermore, the temperament of peoples with trisomy 21 often leads to behaviours of concern which often requires direct and consistent supervision. There are triggers which to us may not even appear as existant, but they are there and when evoked, it can cause a great deal of stress and anxiety for that person and those around them.
The fact of the matter is is that the broader picture needs to be taken into consideration.The person themselves need to become the priority above all else. Why place someone in such a demanding and relentless position of responnsbility when they could live a secure lifestyle, with the appropriate support networks and never have to want for anything. By no means does this suggest that they could not or should not hold a public profile alongside the rest of their family and undertake verious representative roles and patronages, but at least the burden of responsibility would not be so burdensome and kept at a minimal. Being the sibling of a King or Queen would come with more than enough responsibility.
It's about recognising the situation for what it is and in that, doing what can
(and should) be done to assist and empower the indavidual to live as normal and comfortable life as is possible. The pressures of state and community expectations for a head of state would be a significant encumbrance.