All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We are not aware of any specific evidence about the excellence of having a child with special needs. However, we have already issued Fatwa 84468 about the excellence of nurturing children and a person who does so gets a great reward especially if the child is handicapped.
On the other hand, handicapped children are like any other children. If they die as Muslims, then their fate will be Paradise. However, the ones who die after reaching the age of puberty and while being sane, they will be held accountable for their deeds, and if they are mad or idiots, they will be tested on the Day of Judgment; for more benefit, please refer to Fatwa 82190.
As regards the statement that they will become angels, then we are not aware of any evidence about this.
Allaah Knows best.