The challenges of the nation's largest transitional kindergarten

California is in the middle of an ambitious plan to offer transitional kindergarten (TK) to all 4-year-olds by the 2025-26 school year. It's poised to be the largest free preschool program in the country. A lot of the kids heading to TK are dual-language learners. KQED's Daisy Nguyen reports on the challenges California faces as it tries to add a new grade to its sprawling public school system.