Answer
Yes, foreign children can enrol in school in Uruguay even without Spanish proficiency. Children of foreign nationals arriving in Uruguay are admitted, although instruction is in Spanish.
This means that not knowing the language is not an insurmountable obstacle to starting school. But the child will still need to adapt quickly to a Spanish-speaking environment: understand their teacher, communicate with classmates and get used to school vocabulary.
Parents should plan for an adjustment period in advance rather than expect the child to perform immediately at the same level as local children. Regular Spanish lessons, steady communication with the school and support at home help, especially during the first months after enrolment, when the workload is most demanding for the child.