Since I wasn't sure if reading in the dark is bad for your eyes, I decided to do a bit of research to find the truth. It took a 30 second Google search to learn that one shouldn't worry if they consistently read in the dark. Reading in the dark can strain your eyes and potentially cause headaches, but it doesn't weaken your vision.
So let your kids read in the dark if they want. I find it weird, but there's no damage done, so no worries.