Yes many are free "Book 1" books of a series (which I never really like) but there are a few gems to be found and plenty of children’s books to keep them entertained.
The Nikola Tesla one is at https://www.amazon.com/Nikola-Tesla-Imagination-Invented-Century-ebook/dp/B00CATSONE/ref=sr_1_158?dchild=1&qid=1585474231&s=digital-text&sr=1-158 with a 4/5 rating by 7,878 reviewers.
If you’re looking for something to read while being stuck at home, authors and publishers have you covered. There’s currently this list of 420 free Kindles