Hi Flyingmummy
We've had this for ages now and have thrown the towel in. We tried black-out blinds and everything suggested here but our kids (boy - nearly 6, girl nearly 4) are just plain early risers.
We tell them to play quietly in their room or they are allowed downstairs to switch the TV on but they are NOT allowed in the kitchen or any other rooms - if necessary, put catches on the doors to any other rooms to be safe.
They do need to use the bathroom, so we've moved lotions and potions out of reach and each night, before bed, we tighten the taps to the point where the kids can't turn them on - we had a near flood one morning and it was a pretty hairy experience!
We, like you, thought the later to bed thing would work but they just woke up at the same time and were tired, grumpy gits for the rest of the day.
I long for arsey teenagers who stay in bed all day!!!!! Suits me just fine!
Don't dispair Flyingmummy, just work around it - if you and partner are grumpy AND the kids are tired, you're in for some rubbish weekends! At least if you and partner are getting the right amount of sleep, you can keep your cool much easier with the children when they do that mid-afternoon 'droop'.
Keash