Here’s one article.
I really liked this quote from the article: "Jane Yolen, a prize-winning children's book author, contends that "celebrity children's books eat up all the available oxygen ... I have over 250 books out, have won a great number of awards within the field, have been given four honorary doctorates for my body of work, but have never been on 'Oprah' or spoken to Katie Couric or gotten a $100,000 advance for my work.
"I am not complaining. I do very well by the ordinary parameters of the field. But I have been thinking about getting out my pointy bra and brushing up on my singing and dancing because there's no good pop music out there. And because -- you know -- if it's celebrity they want ..”
And here’s another article about the same subject. And a snarky (and funny) blog post by someone who studied celebrity’s books. [Full disclosure: I would guess that many celebrity children’s books are ghosted.]