I've just found out that my ex has been getting a mate to drive him and the DCs from school back to his house in a van. DD1 goes in the front with a seatbelt but according to DD2, she and ex just sit in the back on the floor 
Ex lost his license for drink-driving a while back. He only sees the DCs for a few hours a week.
How do I handle this? I'm trying not to phone ex up and call him a stupid, lying twat. I could send a letter but it wouldn't get there before the next time ex is supposed to pick them up from school.
Help!