I can't recommend Amazon FreeTime Unlimited (now called Amazon Kids+) enough! If you buy one of the Amazon kids tablets (like the Fire 7 Kids Tablet or Fire HD 8 Kids tablet), they come with a year of FreeTime Unlimited/Kids+ included so the tablet pays for itself. The Kids tablets also are covered so if the kid breaks it, Amazon will replace for free but they're pretty hard to break with the case on. FreeTime/Kids+ provides access to most ebooks on Amazon. There are also tons of educational videos, games, all sorts of stuff to help kids learn for any age. With FreeTime, you can set the ages of the kids and completely control everything they access. You can even set limits like they have to read for X minutes before they can play any games, stuff like that. I got 3 kids Fire tablets for 3 of my nieces and nephews and I think everyone with kids should get these. I wish I had known about these sooner.
https://www.amazon.com/Fire-7-Kids-Edition-Tablet/dp/B07H8WS1FT
cheap ~$99, durable (has a case), no questions asked replacement for 2 yrs. My Toddler has this one it works great for her needs (mostly watching netflix and youtube kids). If she ever breaks it amazon will replace it.
edit: if you want you can sideload the play store so you arent restricted to the amazon eco system, but for your needs it probably wont matter. Netflix and youtube are on amazons market now.