What is the book that the girl goes back in time and there's kids running around in the playground singing?

The book you're referring to is likely "The Time Traveler's Wife" by Audrey Niffenegger, but it could also relate to "A Wrinkle in Time" by Madeleine L'Engle, where time travel is a central theme. However, the scene with kids in a playground singing might specifically evoke memories from "To Kill a Mockingbird" by Harper Lee, where childhood innocence is portrayed. If you have a specific scene in mind, please provide more details for a more accurate identification.