When the new monarch is too young the next suitable person in the line of succession is appointed Regent. But William's kids are too young, Harry doesn't want it, his kids are also too young, and Andrew is a pervert.

So it would go all the way to Beatrice at number nine on the list. Most people don't even know who she is, so I think they'd probably call it quits.

There is never a doubt about the line of succession, it is William, then his 3 children (aged 9, 7, 4).

If there are any disasters, making a 9 year old next in line to the throne voluntarily sounds like it.