Wow... I would talk with a pediatrician. Your kids are growing and they need nutrition that they wont get from a vegan diet. If they dont get all the nutrition that they are required it can stunt their growth. What health purposes do you have that made you go vegan? If you dont mind me asking.
You can start it slowly by excluding meat products in their meals, give them vitamin supplements and make another timetable replacing meat products with cereals and other vegetables. However, you need to make sure that they still get a balanced diet. You can talk to a vegetarian expert for details.