I am sure that the plans for the death of Charles and William include a provision that should George be under 18 a new regency act will be required.
Now it is unimportant because George is not the heir and there is time to sort something out.
It would be preferable not to have to take emergency steps when he ascends and not to have a potential power struggle so I expect that as they did when Queen Elizabeth ascended to the throne with a underage heir they will consider it at the point they have an underage heir.
There is no point getting into it now as the situation may be very different in say five years time.
Regency doesn't have to follow the line of succession, that is just the default if no alternative is specified. The government can do what they think best in the circumstances, as they did in 1953 when the idea of Princess Margaret as Regent was worrying so they appointed The Duke of Edinburgh.
What is done is likely to depend upon many factors, popularity, relationships, public opinion, age of George and age/health of possible regents.
They could specify a joint regency or several alternatives.
I think if William reigns first then Catherine as Dowager Queen would probably be a no-brainer as regent but if William predeceases Charles a joint regency between Catherine and Edward or Anne (more likely Edward due to age) would be likely.
Luckily it is all theoretical though!