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.

Suggest setting up a fixed bedtime routine - 6pm dinner 7pm play 8pm start getting ready for bed 830-9pm should be in bed, limit responding to their requests, to make it less interesting for them to stay awake. there should not be a TV in their room, if so, it should be off. No computer / iPad playing in the evening Ensure that if they take an afternoon nap,, they are up by 4pm You will likely have to enforce this structure consistently over one to two weeks, but children this age do respond easily to a new structure once it is enforced. Good luck!