In the future, ereaders will probably take over for all grade levels. Yet the future is unlikely to be here for a couple of more decades because there are a number of issues to overcome. These include things like usability, cost, availability of resources, and technical proficiency (of children and teachers).
Another thing to consider is that kids like going to the library. Part of the reason may be the tactile nature of finding and reading pbooks, but they also like the change of scenery: walking there, exploring a new space, having a space that is geared towards imagination rather than learning.