(rewriting this after seeing Courtenay's post)

I agree that the aunt and uncle who Digory and Mabel are staying with don't count as 'distant' relations. 

I have also remembered a common feature in children's stories from early to mid century (E Nesbit to Noel Streatfeild and Enid Blyton):
some children (often two boys and two girls, all siblings) go to an unfamiliar place to spend a few weeks while their parents are unavailable to look after them (for different reasons). It is is a growing and learning experience, and they usually interact with others where they are staying; there is often a play or show production involved, or a mystery solved.

Why were their parents unavailable?  Working overseas, on a special holiday, one ill and the other taking care of them, etc. If the parents worked overseas, it was a given that the children went to boarding schools in term time, and stayed somewhere else in the holidays. They were always middle class children.

The adult responsible for them was often someone they hadn't met before, someone eccentric, older but fun, challenging them, helping them learn and mature. Occasionally (Nesbit) there is a magical element or even a spiritual one (Susan Cooper 1970s, or John Masefield 1930s).