Never at a Loss for Suggestions
Being smack dab in the heart of the country helps quite a bit - you have a plethora of opportunities within 6 hours of downtown St. Louis. To list just a few:
North: Up the Little Dixie scenic byway along the Mississippi to Hannibal MO and Nauvoo IL. Besides the historical aspects, there are a number of wildlife refuges, working locks, and state parks. Return down the Illinois side of the river and cross back into the St. Louis area by way of the Golden Eagle Ferry.
South: Some great destinations in the little riverfront towns of Ste. Genevieve (French heritage), Cape Girardeau (waterpark), Cairo (river junction), and then on to Land Between the Lakes in Kentucky.
East: Head through Lincoln Country in both Illinois and Kentucky to caving and hiking in the western foothills of the Appalachians in Daniel Boone National Forest.
Southwest: The Ozarks and Branson make an obvious family destination.
All of those are within 6 hours of St. Louis and the sites I've listed are just the tip of the iceberg. See which of them appeal to your kids and then have them start finding more places that pique their interests in the chosen area.
AZBuck