Supposing Charles dies from his cancer. William dies in a terrible boating accident while hunting chukar on the mansion's grounds.
The current succession is this:
1. The Prince of Wales

2. Prince George of Wales

3. Princess Charlotte of Wales

4. Prince Louis of Wales

5. The Duke of Sussex (Prince Harry)

6. Prince Archie of Sussex

7. Princess Lilibet of Sussex

8. The Duke of York

9. Princess Beatrice, Mrs. Edoardo Mapelli Mozzi

Harry won't be allowed to accede to the throne at any time in this century. This leaves William's kids. They've GOTTA have a King.
Parliament will form a Regency, as the kids are too young.
Who will form the Regency? IIRC they must be Royal blood; Camilla and Kate are ineligible unless in an 'advisory' role.

By law, when the King is a minor, the Regent is the next in line of succession who has reached the age of 21. If they don’t want Harry, they’d have to disqualify him on the grounds of not being domiciled in the UK (in fact they might be legally obligated to do so, though he’d probably fight that)…and even then it goes to Andrew, which is almost as bad. It would be a disaster.

What, he has to step down when the crown prince ages up?

That’s correct. The Regency ends by law when the King turns 18. For better or for worse, gone are the days of power struggles over the throne.

Interestingly, not only can’t the Regent refuse to relinquish stewardship of the Crown, he also can’t refuse the post of Regent. So in your example, if Harry wasn’t disqualified based on domicile (which they would almost certainly try to do), not only would he be Regent, but he would have to be. The only way for him to refuse the post would be to relinquish his right to the throne and thus remove himself and his progeny from the line of succession.

Choose a Rothchild.