The simple answer is that there isn't a simple answer. How's that for evasive?
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Children's book illustration can range from relatively inexpensive artwork through to several thousand pounds for a complete picture book. The price depends on the illustrator, the complexity of the artwork, the number of illustrations, the amount of character development required and whether the illustrator is also handling the book design and preparation for print.

It's worth looking beyond the price per illustration, though.
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A picture book isn't simply a collection of individual drawings. The characters need to be capable of telling the story. Sure, you can create a really cute character that everyone is smitten with, then remember that he needs to be brushing his hair throughout the story, but you gave him short arms
that don't reach his head... and he's bald!
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Then the characters you create have to remain consistent from page to page, the illustrations need to work together as a story, the page turns need to make sense, and the finished artwork has to work with the text and the physical format of the book.

My preferred approach is to quote for the complete project rather than simply charging for a pile of pictures. Though, if that's what you require and you have the skillset to build them into a book, then who am I to argue?
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My standard picture-book package is £4,000 for a 32-page book and includes character design, story thumbnails, front and back cover artwork and design, all the interior artwork, text layout and book design, plus handwritten text for things like onomatopoeia, conversations or simply for artistic flair.
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Oh, and I work for hire, so when I'm paid in full, you are given the complete worldwide rights in perpetuity. That means you can use the artwork for marketing material, merchandise, or anything else you like... forever.
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Bill Watterson, the creator of Calvin & Hobbes, reportedly refused to license any of his comic strips or characters, and it was estimated that he effectively rejected up to a billion dollars. I think that's freaking awesome!
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But imagine what you could do with the art from your book if it gets some traction. With me, you keep it all!!!
 
Every illustrator works differently, so when comparing quotes, I'd recommend looking at exactly what is included rather than simply choosing the lowest number.