Reading is reading.
No matter what kind of book your kid wants to read, if they want to read, they're doing better than alot of kids. Lots of kids hate reading. Partly, because of school(if it's assigned they want nothing to do with it during their free time) and partly because they have yet to find a book that interests them. If your kid is reading books that are at a lower reading level than their age, don't make them feel bad about it! Reading ability levels can be a sensitive subject for some kids. I read some books now that are in the Ages 9-12 section. They are definitely below my age appropriate reading level but they're great and I enjoy them!
Please, please, please, read to your kids when they are babies, just learning to read, and even after they've been reading on their own for a while. Not only does it let you spend time with them, it helps develop their imagination, their vocabulary and so much more! Instill a love of reading and books in them when they're young!! Keep at it and they'll start to want to read it themselves, when they're ready. Kids don't learn on your schedule. Keep encouraging them and exposing them to great books and they'll get there!!