Oxford Owl is a useful website related to reading and there are free E-Books that you can read online with your child. Click here for help using this website.
Khan Academy https://www.khanacademy.org Especially good for maths and computing for all ages but other subjects at Secondary level. Note this uses the U.S. grade system but it’s mostly common material.
BBC Learning http://www.bbc.co.uk/learning/coursesearch/ This site is old and no longer updated and yet there’s so much still available, from language learning to BBC Bitesize for revision. No TV licence required except for content on BBC iPlayer.
Futurelearn https://www.futurelearn.com Free to access 100s of courses, only pay to upgrade if you need a certificate in your name (own account from age 14+ but younger learners can use a parent account).
Openlearn https://www.open.edu/openlearn/ Free taster courses aimed at those considering Open University but everyone can access it. Adult level, but some e.g. nature and environment courses could well be of interest to young people.