Hi Jane
I've got a similar situation in my family. In 1911, my father, Fred, was 2 years old and living with a paternal uncle and family in Middlewood Road, Sheffield. His twin sister Gladys was living with her parents and her 11-year old older brother Albert in Dixon Place, Sheffield. A fourth child, Florrie, was 7 and she was living with her maternal grandfather's family in a public house in Clarborough, Notts.

I just have to assume that either the parents were very poor and couldn't feed/clothe all the children or the mother was ill and couldn't cope with them all. I'll never know. My father died some time ago and obviously had an unhappy childhood so I never asked him about it. His mother died in 1920 when he was only 11.
Good luck with your research
Maureen

Jane, there are many possible reasons. Every case will be different and to come up with a possible reason we need to understand all members of the family and their situations.
Can you please be more specific about the individuals?
Dave