Do You Need an ISBN for your children's books?
No and yes.
NO. If you only plan to sell ebooks of your children's titles, and you only plan to distribute to trade ebook sites, then you don't need ISBNs. Kindle, Kobo and Apple don't require and ISBN to publish.
However, because this is about children's books, you should also be selling to the education market. All of the education distributors, as opposed to those trade markets, do require an ISBN.
Each ISBN is assigned to one and only one book of a certain format. That means you need separate ISBNs for each ebook, paperback book, hardcover book, or audiobook. If you publish one title in all four formats, you'll need four ISBN for that one title.
How Much Will You Pay
If you live in Canada, ISBN are free. Other countries have their own method of assigning ISBN. In the U.S., Bowker.com is the provider of ISBN numbers.
If you look at the Bowker site (or myidentifiers.com), the US provider of ISBN, the advertised prices are as follows?
1 ISBN is $125 or $125/ISBN 10 ISBN are $295 or $29.50/ISBN 100 ISBN ar $575 or $5.75/ISBN 1000 ISBN for $1500 or $1.50/ISBN
Obviously, the most economical is buying a larger block. But ISBNs are like any other product: they go on sale. You need to look for sales to buy a block. Different writers' groups include periodic sales.
For example, if you're a member of the Independent Bookseller's Associaton, one membership benefit is 15% off on ISBN and other services from myidentifiers.com. That puts 1000 ISBN at $1275 or $1.275/ISBN. And there are often promotions lower than that.
In 2013, through a one-time promo for members of an indie writer's group, I bought 1000 ISBN for only $850 or $0.85/ISBN
For me, buying a block of 1000 ISBN meant that I was all in on my new business venture. And it's been a blessing because I don't hesitate to use an ISBN. When I published audiobooks, one education distributor I was interested in is Findaway, which requires ISBNs because they distribute to bookstores who need it. Of course, if you publish on ACX, Amazon's DIY platform for audiobooks, you don't have to use an ISBN. But because I expected to use education distributors, I used an ISBN. If I had only bought 10, it would be harder to "spend" that ISBN, since they can never be reassigned.