If you publish on Smashwords.com, they’ll put your book in all the important places. You could actually get by just fine by only publishing with Smashwords. Your book will then be listed on Amazon Kindle (that deal still hasn’t happened as of 6/4/2012), iBooks, B&N Nook, Kobo, Sony, etc. Just keep in mind that Smashwords is a business and they’ll get a meager cut of your royalty.
Publishing on Amazon for Kindle will put your work on the most popular eBook outlet in the world, by far. You get the full 70% royalty (35% if your book is priced at less than $2.99). If you publish on Smashwords and they place it on Amazon for you (if and when that becomes a possibility), your royalty will be about 15% less (60% on a book priced at $2.99 or over).
B&N for Nook is coming on strong, but still miles behind Kindle. With the Nook having a number of options to choose from, if you have a color children’s book you might consider publishing it on B&N yourself, rather than going through Smashwords where you’ll have to give them a cut of your royalties.
Then, there are the “others” (Kobo, Sony, iBooks etc.) Here again, you can cut out the middleman (Smashwords) if you go directly to these outlets—but, for the relatively few books you sell, will it really be worth the trouble? And there might be some additional hoops to jump through like Apple’s “publisher” requirement.
One you don’t have to consider, anymore: Google Books. They’ve shut this program down. From my experience, it was extremely buggy and not well maintained, anyway.