Cambridge University Press is making higher education textbooks on its Cambridge Core program free to access online through May 2020 due to Covid-19. You can browse the more than 700 books available at
Also from the Cambridge University blog post at https://www.cambridge.org/about-us/covid-19/ :
English Language Teaching, in partnership with colleagues at Cambridge Assessment English, are producing free, online content for teachers and learners. Our World of Better Learning blog is supporting distance learning for those whose schools have been closed. Daily blogs with text, audio and video content focus on different aspects of online teaching and we are making our content accessible to the maximum number of people possible.Source : https://www.cambridge.org/about-us/covid-19/
There is also a Bighter Thinking Blog at https://www.cambridge.org/gs/education/blog offering tips and advice from teachers who have been affected and guidance for those facing or experiencing school closures. So this also could be a useful resource for educators and parents looking to educate their kids at home during this period.