My 3 children has been sleeping very late. As late as 3-4am. Now the youngest joined in. All of them below 5years old. I am really exhausted. I have no idea how to readjust their body clock.

Make sleeping a fun affair! Arrange for an indoor camping activity and have everyone sleep in the living room. In the day time, have lots of activities to "tire" them out and by night time, hopefully they will be tired.