If they really wrote the books themselves, I'd consider them unqualified to write them. But I seriously doubt they even did that. They had an idea, and someone was paid to flesh it out, because the publisher knew the celebrity name would sell it. If you think it through, is there any reason in the world to want your child to read a book "written" by Madonna? Um, no.
Selling children's books has always been one of the very hardest roads for authors. Really, really hard even if you have ins with publishers. So these instant books make it even harder for legit authors. Thanks, Famous Person with No Talent.
Not that every children's book that gets published is wonderful; many are horrible. Assume these celebrity books are the worst of the worst and pass them by. Chances are, your children won't be missing a thing.