Most likely they're gotten from kids at school or from playmates. You need to get all the nits out or they will develop into lice. Wash bedding, hats, etc.
THe most common source where to catch head lice is at school when your kids are playing with other kids. The best way to keep them away from your kids is to wash there clothes very often and to explain them how to avoid being in contact with other kids heads.
Head lice are spread from contact with other people who have them. If your kids keep getting it, you are probably not cleaning and hair-combing thoroughly enough.
They can get them from other kids at school or even at home. Wash their hair with special shampoo as well as pillows, sheets and blankets. dry on high heat
Children get head lice from other people, usually other children. They are transmitted either from direct contact or via an object, such as a hat, that has lice on it.
Head lice can be transmitted from person to person, so they could be picking them up at school. The other possibility is that they are getting them from their clothing or bedding over and over again. Make sure you thoroughly wash all of their linens and clothes.