A lot of John Lewis trousers have adjustable waists. For a long time, Boden sold the only jeans and leggings that fit my DD, but she's branching out a bit now. The fashion for skinny jeans fading has helped. She has a pair of Gant jeans I bought in the sale in desperation - I would never spend that much normally - but they fit like a glove.
Joggers for school are the things I find most difficult. Even a functional draw string doesn't pull in enough to stop them falling down.