I just finished the first of a series of children's books and ready to sell. How does one sell their book without giving up all rights to it?
I think the key question is how long do you grant publishing rights. You should set some kind of limit and some kind of process for rights to revert. Any legitimate publisher will spell that out in the contract (and might even specify early termination clauses).
I'll warn you though, publishers hold a lot of cards and might just present a take-it-or-leave it contract.
If you're doing a series, you might have to give up a lot on the first volume or two and then negotiate later for better terms.
Another thing. You might be talking about not selling digital rights or movie/TV rights. That's possible, but that might cause publishers to balk at your project.
A better way to negotiate that might be to require final approval when negotiating movie rights. (Typically though, if a publisher can get something in other mediums, that's a good thing for the author in more ways than one).
Traditionally, your rights revert to you after a specified period of time when the book is out of print. However, be cautious! In recent times, publishers have been able to circumvent this by placing books into perpetual "print-on-demand" limbo. If you don't want that to happen to your book, make sure you cover it in your contract. That's one of the main reasons it's good to have an agent, even if you can sell your book yourself.
However, you may be putting the cart ahead of the horse. Most writers face their biggest hurdle in getting someone interested in publishing their book, not in protecting their rights to it.