When I was in college I took a lot of children's lit classes. As many know I have a love of kid's books. I have just a few... hundreds. This is a good thing since both my boys love literature.
One thing that they stress in these classes is that children should read good literature. Often they say that you should not read/buy books that have characters from tv shows and such. When I was a teacher, I did agree with this for the classroom. All of the books that I read aloud to the class, were considered good literature.
There are some books out there now that they have based children's shows off the books such as Author, by Marc Brown; Frankin, by Brenda Clark and Paulette Bourgeois; Olivia by Ian Falconer and many more. Most of these shows are actually really true to character of the books.
When children are learning to read, you have to find books that interest them. You have to make a connection to thing that they know to get them involved in the story. So, what is the easiest way to do that? Get characters that they know. My sons have Cars stories, Thomas stories, and many others based on the characters they enjoy watching.
Do I think I'm doing a disservice getting these books... nope. I think you need to do a combination of both. So the next time you get books for your children try picking up one that is based on a character or theme your child is really interested in, and one that is not. This way you can engage them in both types of literature and literacy!