Just to add to what Chas has said:

For William to "take over", Charles would have to abdicate first if he is still alive when Her Majesty passes. It would then require a parliamentary bill to be passed to give effect to this desire to which he would have to give his Royal Assent.

The consent of every country where he would be Head of State would also be required.

There is no precedent for "jumping" the line here.