Anna Weinberg of the Book Standard examines both sides of the publishing phenomenon of actors/sports figures/politicians/and other celebrities writing children's books.
Do the sales from these books allow publishers to find and support new authors? Do celebrity books bring attention to important issues? Are they poorly written public relations stunts to bring attention to a publisher's back list and a celebrity's vanity?
... access isn’t the only thing that celebrities get. Jane Yolen, an award-winner many times over, who’s published more than 200 children’s and YA books since 1961, points out that “celebrity-authored books get high advances—usually about ten to twenty times higher than the average children's book author” (which is often about $5,000 for a debut novelist). “[They get] enormous ad budgets—about 100 to 500 times the average children's book author's—and eat up all the available oxygen at the publisher's.”
Weingberg fairly addresses both sides of the issue in this excellent article.
The question that has been debated on the Child_Lit listserv is why children's books? Celebrities could write cookbooks, decorating books, novels, self-help tomes but they are all choosing 32 page picture books. I doubt many of them see themselves as the next JK Rowling but Harry has proven that there is serious money, respect and fame to be had writing for children. Children's books must be serious business. Also, 32 pages isn't so long, how hard can it be to write one?