That's a big question.
If a child were born unable to carry out the role of Monarch, I think sense would prevail and the next in line would take precedence. I don't know what the law says.
To complicate the issue .. What if an heir were to develop an ailment later in life that rendered them incapable of working? A major accident or dementia?
Would their brother or sister take their place, or their own infant child?
Which brings me to a final question .. What if an heir were born with a major disability, unable to take up the top position but later in their life they had a child. Would that child of the disabled heir have any rights in the line of succession?
Consideration would have to be given to any discrimination due to a handicap.
The disabled heir would have to be properly diagnosed and assessed as to being unable to take on the role of Monarch. That would be a tricky situation. I wonder what the law dictates??