My kids wake me up every week-ends

Every week-ends  it's the same story... Around 6:30 am my kids (4 and 5 years old) wakes-up , they start to play together and they wake me up around 7 am, because of the noise or because they argue with each other... I wish i could sleep until 9am sometimes. So basically I would like them to stay in bed until 8:30 at least... But they don't know how to read a clock yet. If someone has an idea to make them sleep or stay quiet longer please let me know.