Six Flags is good for a day's outing. There is a lot to see and do. However, I have to admit something up front, I hate roller coasters! Several of the rides are not suitable for the younger set, but I do have thrill-seeking children and one crazy husband. They love the thrill rides!
There are a ton of things to do while you are there. My favorite area is Hurricane Harbor. There are water slides and tube rides as well as a play place for kids. My family likes the Lazy River to tube in, but when the park is busy, you can only go around one time, then you have to give up your tube and get back in line. Hook's Lagoon is a fun place to play with your kids. It consists of a five story tree house and eight water slides. You have to be less that 48" for the pirate rides and at least 42" tall for the slides. This area also has a 1,000 gallon water dump bucket that that is awesome on a hot day. We also really enjoy Hurricane Bay. It is a 30,000 square foot wave pool. Only rental tubes are allowed in this pool, and the pool is cleared every hour for several minutes for a safety break. If you go to Hurricane Harbor, just make sure you are wearing swim wear if you want to do the slides or any of the other rides, as they won't let you on in street clothes.
Another favorite area is the 1904 World's Fair, which is home to the Colossus: an 18 story Ferris Wheel. You can pretty much see the entire park from the top. Very cool.
The Warner Brother's Back Lot area has a couple of roller coasters that my husband loves. One is the Batman: a huge inverted (yep, upside down on this one) steel roller coaster. Another one that he likes is the Ninja. Six Flags calls this the black belt of roller coasters, it has a double corkscrew among other things. My daughter's favorite roller coaster is the Boss. It is a 122 foot tall wooden roller coaster featuring a 150 foot first drop, and a 570 degree helix. She says this is an awesome ride! It isn't in the Back Lot area though.
If you want to ride a water ride, but forgot your swim wear, you can ride the Thunder River, which is a white water rapids boat style ride in the main park. You have a good chance of getting soaked on this one.
My favorite ride as a kid was Tom's Twister: a centrifugal force ride where the floor actually drops from beneath you leaving you suspended mid air!
Other fun things to do include the Speed O'Drome: go carts at additional cost. Must be at least 58" to drive. There is a ride the is similar to a bungee jump, where you are strapped in and launched from a 152' tall tower and free fall about 50' feet before swinging pendulum style. I don't know the particulars on this one, but I think the cost is somewhere in the $25 ranger per person. I have never done it, but it is very cool to watch.
A couple of other rides that bear mentioning are the Log Flume, which is a canoe style ride fit for the entire family, and the Mr. Freeze: a roller coaster that runs forward and then in reverse. Not for the faint of heart -- this one pulls 4 Gs! There is also the Screamin' Eagle: a 110' tall wooden coaster. This one is good because there is seldom a long line.
One of the two areas for little kids is Looney Tunes Town and the other is Bugs Bunny National Park. Looney Tunes is an older kids play area. There are some rides for preschoolers there. Bugs Bunny National Park is new, I believe this year. It is cool, because it has rides suitable for little kids, but they are large enough that older siblings or even parents can go on them -- and, yes, enjoy them. There is also a huge climber play area.
If you plan a trip to Six Flags St. Louis, be warned that food inside the park is expensive. However, you can leave the park to eat elsewhere and return. My advice is to bring a cooler and leave it in the car though; it is too expensive and too crowded to eat in the park. You can get your hand stamped to leave and re-enter. Some people even bring the little bbq grills and cook out on the grassy areas around the parking lot.