I woud suggest that you monitor them while they read for long periods so that their posture is good and that they take periodic breaks to focus their eyes on objects that are far away so that they do not get eye strain. Beyond that, they should be fine. My son is a bookworm in phases - he will not put down books for a few months. Then, he will be off doing other stuff for a few months and then, he will be back to reading night and day again. I encourage him to use a Kindle when he spends many hours on reading - he increases the font size to large for reading for several hours.
