These seem to keep their attention more because they are interested in the book. Make sure they are age appropriate.
Some books have a level on the back of the cover.
Also a general rule is whatever age the main character in the book, it is usually appropriate for that aged reader. Remember, these assumptions are made for the average reader, and your child may be above or below average. When you are trying out a new book, if your child can read a page of the book with less than three mistakes, it will most likely be an appropriate level. It is important that your child not get frustrated when reading. It should always be an enjoyable experience.