I haven't got any kids, but I'd like to try to make a contribution to this thread if I may.
I think that if I had a kid, I would try to teach them that the world is always more complex than they think that it is. So they should practice staying curious and paying attention to their surroundings.
The other point that I thought of is that young kids, like the 3 - 7 year olds that you are writing for, often learn better through play than through more formal learning. So if there is anything that you can do to add gamification to your book, it might help with the learning process.
For example, it might be possible to get the kid to do something as they read the book in order to score points and so at the end of the book they have a total. If their score sucks, then they can just read the book again and get a better score.
Another possibility, although a lot more difficult, would be to add a website to the book and to put some kind of interactive experience on it in order to increase the game possibilities. So the kid had to read something in the book to do something on the website or vice versa to get the points.
Another option might be to teach the kid a skill in the book as a lived experience of say for example a growth mindset. So you might start by saying, "I bet that you can't juggle three balls / beanbags at once" or "I bet that you can't juggle two balls / beanbags with one hand" and then by the end of the book, they would have been encouraged to practice their skill so they can do it. Juggling was just an example that I thought of, it could be anything a bit unusual.
For example, if I had a young child, I'd like to teach them the concept of a growth mindset, versus a fixed mindset.
Sorry MJ, but this quote is bugging me a lot. AFAIK, young kids naturally tend toward a growth mindset anyway. So it seems to me that a fixed mindset is what people learn as they get older and they build up limiting beliefs and experiences. So I think that you could consider encouraging kids to hold onto their growth mindsets rather than having to teach them what a growth mindset is. If you don't believe me, then consider, how many eight year olds do you know that are the same as when they were four? Growth is all that young kids have known. Their bodies are growing and their knowledge and skills should be growing at school.
I'd like to let them know that their current circumstances doesn't need to be their future.
This is also bugging me, because kids have very little freedom. Like if you are a kid and you want to do something and your parents say "no", then that's it. You can't do it because you are not allowed. For example, if you wanted to move to another state, you could just go, but if you wanted to move house and you were ten years old, then the likelihood is that your mum wouldn't let you and that would be final. Your dream would lie in tatters, whether or not you had a growth mindset or whether you had read an inspirational book or not.
For example, it is a legal requirement in the UK for kids to go to school, or to have equivalent home schooling. If you are are kid and you want to quit school because you think that it sucks, you can't without you or your parents getting into trouble with the law and the police.
The amount of freedom that we get by leaving home is phenomenal and your prospective readers likely won't have that yet. So if you inspire kids to go and do what they want, then likely quite a lot of them will just get into trouble and your book will get lousy reviews saying "this book encouraged my child to misbehave, do not buy it."
I hope that this reply is useful to you and that you don't find it awkward. I understand that you might be annoyed by my not fully agreeing with everything that you say. Feel free to delete this post if you don't like it.
I wish you every success for the book.