。
She had the children get ready for a walk.
Literal
She [topic-は] children [target-に] walk [genitive-の] preparation [object-を] made-do.
支度をする ('to make preparations, get ready') is one of those quintessentially Japanese verb phrases — built from 支度 ('preparation, getting ready') + する. Used widely for getting dressed, packing, prepping a meal, or readying oneself for any planned activity. Compare 準備 ('preparation, setup'), which is more formal and broader; 支度 carries a domestic, immediate-task connotation. The causative させる makes the children the doers of the preparation, with に marking them as agents.