Sounds like a hardware/firmware issue.
Contact Pocketbook support. There should be a link wherever you bought the 602 from.
My experience has been that they are very friendly and good at what they do.
In the meantime, take off the cover and disconnect the battery to see if that resets it.
On the children's books side, for classics, you might want to check out Munseys.com. They have a section of kid-rated ebooks in pretty much every mainstream format:
http://www.munseys.com/detail/mode/cat/20/Children
Hope this helps.