Thank so much for all the great advice

It sounds like Fenton would be an okay place to stay so now I just have to decide on what to do there. Magic House is at the top of the list (along with the brewery for the hubby).
I'm debating the zoo, too, because I live in Louisville and we have a pretty nice zoo but our Science museum is horrible. I thought Grant's Farm sounded different so that is why I wanted to give it a try.
I am debating the city museum just because of what I read on another thread. It talked about kids getting lost easily and hurt. My husband would have a heart attack over this!
Is the Science museum any good? I saw they had a special body exhibit. My kids are really fascinated with that type of stuff but not sure if this one would be appropriate.
Former St. Louisan here. I spent a lot of my undergrad in that zoo doing research, and let me say that it is a WONDERFUL zoo - and free! Also, it is near the Planetarium, Science Center, and Museum, and all of that is housed inside a wonderful public park (Forest Park - although the Science Center is slightly outside of the park, you can walk across a very cool walkway from the Planetarium over the highway to the Science Center). You could easily pack a picnic lunch, go to the zoo, then stake out a spot on the hill and let the kids play while you enjoy your lunch!
The Arch is great, but it really needn't take up more than 2 hours tops.
City Museum is wonderful, but if your husband is would have a heart attack over the kids being hurt I'd suggest you don't go. Even as an adult, I have been hurt in there. HOWEVER, if you commit to going and follow your children around closely, it is a one-of-a-kind area that is so much fun to explore! You could get lost in there for days and not see everything; I've been quite often and met one of the welders there, and I also was a chaperone when my nephew's class went. VERY COOL, and worth the cost. Here's my suggestion - if your kids will listen to you in public, go. If they tend to kind of tune you out when you're someplace new, don't go. That makes all the difference in whether a kid gets hurt or not at City Museum - IMHO.
I haven't been to the science center in years, so I can't comment on if it's still as cool as it used to be. I used to take my nephews when they were younger - they LOVED the big dinosaurs downstairs! There's a good food court (not bad prices, not bad food) and there are always interesting exhibits. It costs to park, but if you park in Forest Park sometimes you can avoid the cost - although I'd only park on the road leading to the Planetarium if you're heading to the Science Center, as otherwise it's quite a hike!
As for Grant's Farm - I've never been. People I know who have been say it's only really good if you have a private tour. Otherwise, you might as well go to the zoo.
Finally, I couldn't say enough good things about Magic House. I agree with a PP, though - try to do that during a weekday if you can, otherwise the place is PACKED.