You have a system message, usually a software update, but sometimes just a useless message from Nintendo. Mine was glowing today just to tell me I should join Club Nintendo and earn points for the Wii shop channel. Most every other time it's been a software update, last time it was a Netflix update.
Just turn it on and check your messages. And like others have mentioned, you can go into settings if that blue light bothers you. I leave mine on, its nice to know when updates are available.
It's also possible that your kids are connecting with friends who also have the Wii. For awhile I was getting the blue light every day. Found out Alli was chatting back and forth with a friend who also had a Wii.