First, take them to a family reunion in Southern California. Oh yes, that's pretty fun. Plus, hanging out with the cousins all day for a week will ensure deep sleep for everyone.
But to really wear them out,
take them to Disneyland!
You can try Splash Mountain, but don't sit in the front. You will get really wet. Trust me. I have "a friend" who found out the hard way.
Then look for the Kindergarten class mascot in the gift shops.
Even if your three-year old is tall enough to go on Big Thunder Mountain and begs to go, don't take her. She will not like it. (Not all grandmothers like Tower of Terror either - weird.)
If the seven and eight-year old cousins want to go on their first upside-down roller coaster together, let them. It gives their moms in the car right in front of them an extra bit of adrenaline.
At the end of the day, be sure to have a strong dad.
If he is a superstar that will help.
When you finish your days at Disney, do your homework.