Flintsters
I have three things for you to look up.
The RV Park at Casino Queen RV Park across the river. I am told it is not a bad park, but we seem to go through there when it is not open. But the lot has a great place to dry camp.
Look at maps because that park is walking distance to the Tunerville Trolly that will take you into St. Louis. You will be looking right at the arch and the Museum of Western Expansion. (Plan half a day.) You don't have to fire up the truck and then find a place to park.
As you are traveling with kids, maybe even if not, look up Chahokia Mounds State Park. This is worth a half a day even if you don't go through the grounds. The might be a better stop on the way in as it is off I-55. Most people don't know that there were First People's cities as large as London of the same period.
To be completely honest, Chahokia is what started us paying attention of historic mounds in the country. We were amazed.
Matt