Yes, the royal family should sell the crown jewels for charity, but they should sell them to an art collector who will place them in a museum to retain their historical value. It doesn't make sense for society to horde objects of wealth within upper class families, but it also doesn't make sense to simply sell them to another person who will horde them.
The crown jewels are a historical artifact and belong where they are. They are currently on display, where people can see them. The money for charity would be nice, but their are other ways of raising it that do not involve selling an important historical artifact like the crown jewels.
The crown jewels should stay secure to ensure their survival and well being. If the crown jewels are sold, there is no guarantee these valuable pieces of history will receive the care and attention they deserve and require. There are several other areas from which money could be taken to donate to charity.
The crown jewels have played an important role in the history of the British monarchy over hundreds of years. Furthermore, It should not be up to the general public to decide what should be done with them. Therefore, the long-term fate of the jewels should only be something that the royal family should consider.