OP, in the nicest possible way, when you tell stories to your children or nephews and nieces, you have a captive audience. They know you, they love you, and more important, you know them and what kinds of stories/characters/situations they like. You have them in the room in front of you and can judge when they're getting bored or sleepy, and, most of all, you can do a lot of the work with your voice and face, acting out characters, creating suspense, adding humour etc.
When you publish a children's book, you have none of the advantages that you have in a 'live' situation with a captive audience that is already predisposed to be interested in what you have to say.
You've never met your audience and you don't know who they are. What works with your 'live' voice simply won't transfer effectively to the page, and you will have to do a lot of work to try to recreate the effects, and to create something which will keep a strange reader gripped.
You will need to figure out exactly what age range you are targeting, and what else that reader is likely to have read -- if there's already a hugely popular series out there about Icelandic fairies who train dragons, then you're going to need to come up with something else.
Depending on what age range you are writing for, it may be likely that an adult will buy the book for a child -- you need to stop that adult putting your book back down on the Three for Two table at Waterstones' and picking up something else.
And to even get to your audience, you will need the beginning of your children's book to be so good that an agent's assistant reading through the slushpile late on Friday afternoon sits up, reads it again, and passes it to the agent immediately. Then, once you're signed with an agent, that process needs to happen all over again with an editor who can get your book past acquisitions and marketing and thinks it will sell -- and agents and editors spend their lives reading books in the field you're targeting. Brutally speaking, if you want to publish your books, you need to be better than the competition, which means you need to read the competition.
Obviously, not reading is fine if you are happy to write a book which is only for your children -- but you talked upthread about publishing.