My daughter who was born in 1978 in South Africa wants to immigrate to the UK. She has great grandparents who were born in the UK on both sides of the family. Her husband's great grandfather was also born in the UK. My wife and I (her parents) are now both British citizens for the last ten years. By what means would it be possible for her and her family to immigrate to the UK?
Unfortunately, the UK ancestry visa is only available to those with a grandparent born in the UK, so your daughter and son in law derive no benefit from their British roots.
Family visas are available for children over 18 only if the child remains dependent on the parent, so they cannot immigrate based on your sponsorship.
Their best bet, therefore, is probably the same as anyone else: find an employer willing to sponsor one of them for a work visa.