The second the car rounds the corner and you see the beach below you, you'll know you made a good choice bringing the kids to Petani Beach.
There are two beaches here. The one on the left has the restaurants and chairs for rent. That one is kind of your typical mediterranean beach.
The beach on the right is a smaller, more natural one. There are no chairs or bars on that beach. It's really pretty. Even if you choose the other beach, you should have a walk over to this one.
Both beaches are rocky. So it will be a bit difficult on the feet to get into the water. The water will be really clear though because of the rocks.
The rocks that separate the two different beaches is a great place to snorkel around. There's a good amount of small fish in the water that should entertain the kids.
Parking is free ... but hard here. It's best to come as early as possible to find a spot. If you come a bit later and all the spots are taken, look at the parking lot in a creative way. When we were there, we found a spot on the side of the parking lot. When it was time to leave, there was a single line of cars in the middle of the parking lot that allowed everyone else just enough room to get out of the parking lot.
Last Bit of Advice
There are two restaurants and a bar right on the beach. Have lunch at one of the restaurants. The menu is pretty standard at each place. The food is good though and you'll have great view of the water.