Due to the current situation there is an absolute wealth of material online for free, and this includes eBooks! Here are some of the best resources I've found for free with no sign-up.
Oxford Owl eBook Library: https://www.oxfordowl.co.uk/for-home/find-a-book/library-page/
BookTrust: https://www.booktrust.org.uk/books-and-reading/bookfinder/
Free Children Stories: https://www.freechildrenstories.com/
Monkey Pen: https://monkeypen.com/pages/free-childrens-books
Magic Keys: http://www.magickeys.com/books/