Hi bucksFarang. I assume your Thai family is from around the Si Thep area and visiting is the reason for your trip? To be honest, you're going to be struggling to come up with an itinerary to occupy a couple of kids for more than a few days without travelling a fair bit further afield. Have you been there before, and where will you be staying? My wife's village is about 15kms from Si Thep and we go up there (from Pattaya) every month or six weeks or so and stay for about a week at most. Fortunately I'm quite happy to stay at our house in the village just chilling in the hammock with a good book, some music, a few cold beers etc as there isn't much to do locally. A trip to the Si Thep market on a Friday morning being the highlight of the week. There are some things worth seeing that can be done in a day, but after Si Thep Historical Park there isn't really much else locally (particularly with young kids in mind). There are one or two nice national park type areas within an hour or so's drive. Pa Hin Ngam Park is just over the border in Chaiyaphum Province, scenic but once out of the car it's a bit like hard work for youngsters. Sap langka Wildlife Sanctuary is not to far away and again is nice but to see the interesting bits (waterfalls, and a cave full of bats), you need to walk a fair distance.
In short, there are no amusement parks in the area, but some nice scenery if you are prepared for a bit of a hike. I'll ask my wife if there's anything else she can think of and I'll let you know; her knowledge of the area doesn't extend much beyond the market!