Kate Holt

When I was a girl I read and reread a novel written by two young people that was a children's adventure story. A group of friends go on a journey together with horses, dogs and a pony and trap. They describe their journey to each other as following the course of the river Indus and indeed the name Indus was in the title. Can anyone remember or help me identify the novel and its authors? I'd be so grateful for help.