The Archaeological Site of Olympia is an interesting place to visit with the kids. It consists of two museums and a large area with the ruins.
If you're driving in, the maps app you'll most likely be using will have you drive right through the site, up to the ticket booth. Depending upon the season, you might be able to stop right next to the ticket booth. Someone will definitely come out and tell you that you can't park there. Ask them if you can park there until you get your tickets. They might allow that. It all depends. If it is early in the summer season, they'll probably let you.
Get tickets for both museums and the site. After you get the tickets, you'll have to move the car.
The road that you came into the site on was oneway. Keep driving up the hill and into town. Don't park at the first parking lot that you find. That lot is reserved for tour buses. It's not marked very well, but chances are a police officer will come over and instruct you to move your car.
There's a public parking spot right next to the Kladeos Café.
It's best to see the museums before you walk around the ruins.
Last Bit of Advice
Bring some bottles of water when you walk around the ruins. Chances are good that it will be hot that day. Also, be sure to put on your sun screen. Once again, expect hot, sunny weather.