Private residence relief: dependent relative: incapacitated by infirmity
A relative is infirm if he or she is mentally ill, physically disabled or chronically sick and is unable to support himself or herself by working. But he or she must be over the school leaving age to qualify as a dependent relative.
You should not consider a child who is under the school leaving age to be a dependent relative no matter how infirm that child may be. This is because the law makes it compulsory for a child to attend school. It is the legal obligation which prevents a child from maintaining itself and not its infirmity.