Its a hard thing for the entire family. A lot of high functioning Downs children can go out get jobs and and function pretty much on their own in a sheltered environment and with a little life coaching. Depending on her level of ability, A young lady with Rett syndrome may not need 24 hour care, but will probably require 24 hour monitoring and supports.
Because they are getting older they need to start looking at a social worker for their child and look at support options in the county they live in. (Which hopefully provides support for the developmentally disabled!) The older their daughter gets the harder the transition will be. There are often waiting lists and questions about funding which involve years of lead time. Better to start sooner than waiting. (And they might already involved in this process.)
They have gotten some help from some kind of social services, and have at least one friend who helps and they are able to pay her, and she also is a loving caretaker and much younger.
I’m sure they’ve given the future thought and planning.