@Morgan-Wallace aww, thanks so much! Glad you found it helpful!
Since @RachelArmington already replied with some good answers, I'm just going to add to what she said.
While KDP's offer of the free "use one of our ISBNs!" is tempting -- it's really not something I'd recommend. This is what KDP says about their free ISBN: "The free ISBN from KDP can only be used on KDP for distribution to Amazon and its distributors. It cannot be used with another publisher or self-publishing service." (Direct quote taken from the KDP website.) So if you use a KDP ISBN, you're agreeing only to print and distribute your book exclusively through KDP, for the entirety of the time you use that ISBN. Amazon owns that ISBN, meaning that while you own the copyright to the book, Amazon owns the distribution rights to that ISBN. Using the free ISBN would work for you only if you expect that this book will be the only book you ever publish, that you don't anticipate ever wanting to print or distribute it elsewhere, and if you don't expect the book will sell well. That includes bookstores, libraries, or other brick-and-mortar stores, most of whom will not put an Amazon-printed book on their shelves.
(This might really be 1b.) Another reason not to use the free KDP-provided ISBN comes from the Nonfiction Author's Association: "Some of the print-on-demand services offer free ISBNs, which sounds easy and appealing if you’re trying to keep costs down. However, when you register your book under a free ISBN, it then puts your book on record as being published by the entity that provided the ISBN. This is a common rookie mistake." In other words, KDP would list itself as the publisher of your book, since they provided the ISBN -- but YOU are the publisher of your book, since you're self-publishing. You want to make sure that you are clearly listed as the publisher of record.
Yes, as Rachel said, IngramSpark has recently begun offering free ISBNs to US-based self-publishers. But here's an interesting tidbit direct from IngramSpark: they don't recommend it. Here's what they have to say on their FAQ: "An ISBN is an expense many self-published authors are confused about. If you use a free ISBN with IngramSpark, your publisher imprint will not be associated with your book—it will hold IngramSpark’s imprint, Indy Pub. It may also limit where you can print and distribute your own title. At IngramSpark, we believe it's in your best interest to be recognized as the owner of your work and a publisher in your own right, which is why we encourage publishers to purchase their own ISBNs. You can read more about ISBNs and the benefits of purchasing your own here."
Your book could be picked up by a publisher after it's been self-published. But as Rachel said, it's rare. Don't expect this to happen, even if your book is great. Why? First and foremost, a traditional publisher is a business. They make decisions on which book to publish based on how well they think it will sell. Most self-published books don't sell well. So in the traditional publisher's eyes, if a book has a proven track record of not selling, why would they take that gamble? On the flip side, if a self-published book does sell well, the traditional publisher may be concerned that the author has done such a good job of marketing, they have already saturated the market so the publisher may not be able to expect strong sales if they decide to pick up the book.
But like they said on Reading Rainbow: don't just take my word for it. You should definitely research all of this yourself and learn all you can about self-publishing before jumping in and publishing your own book. It might be a good fit for you, it might not -- and only you will be able to determine that.
If you're overwhelmed with all there is to learn, here are a few places to start:SCBWI -- mentioned them before and will mention again because they're a wealth of information. Membership is around $80 or so a year and with that comes access to webinars, networking with peers (especially within your region), and The Book. The Book alone is, to me, worth the price of membership. This fall, there is also going to be a special edition of The Book just for self-publishers (I'm really looking forward to that!). The Children's Writer's and Illustrator's Market -- this book is updated every year and contains articles about writing for children and the children's book industry. You can preorder the newest edition, which comes out in October. ALLi (the Alliance of Independent Authors) -- Note: if you're a member of SCBWI, they have teamed with ALLi to provide self-publishing information and advice for the children's book market, so you may not need to join ALLi. ALLi does have great articles and has researched reputable services to help serve and protect the independent author community.
Hope this helps! And if you have any questions, please ask. ❤️