You are not silly, but I don't think this need be a significant concern either. It doesn't sound like their reading interferes with other aspects of their development (social, physical, etc.). They just prefer reading for entertainment. I am pretty sure there are a lot of others on this board (adults and children alike) who have had periods past and present when reading was the primary leisure activity. Some of us certainly were guilty of walking-under-the-influence (of a book) for protracted periods of our school years!
