I am coming to St Louis this weekend with my wife and three girls ages 10, 8, and 5 for about three days. We have to take my youngest to the children's hospital there so I will have to entertain my older girls while mom and the youngest make the doctor appts.
 We are looking at the zoo, childrens' museum, and a butterfly exhibit. I would appreciate any ideas esp low cost ideas near downtown. I just have to kill time more than anything else.
 Also, what is the deal in St Louis? I live in Memphis, TN and habitually carry a Glock 19. I don't want to have trouble in MO. I have a TN permit and it looks like it is honored in MO. Looking at the crime stats for St Louis it doesn't seem near as bad as Memphis. Where is the bad part of town so I can avoid it?

I'm a college student at SLU and from my experience, I'd just really stay away from East St. Louis, which is right over the bridge.  If you get pretty far north off grand, it gets a bit sketch, but I've pretty much always felt safe everywhere around town except just over the bridge.  I'm sure your girls would like the city museum off Washington street downtown.  I want to say it’s on 10th and Washington.  It has fun exhibits and you can let them run around there.  Union station is always nice to go walk around.  The science center is neat, that’s right off 64/40 near kingshighway.  I hope this helped a bit.

Most of the attractions in forest park (the Zoo, the Science Center, etc) are free, which is different from most places (ie Chicago)- though Forest Park is actually on the Western edge of the city proper, and not downtown.  I've heard that the City Mueeum downtown is something anyone and everyone loves.

After having lived here for quite a while, I'd say that the only real areas to avoid at all costs are east and north St. Louis.  There are some questionable areas off of grand street too but where you are looking at going (zoo, etc.) you should be just fine.  The attractions you picked are excellent choices and you should have plenty to do there for a few days.

If you happen to get some time to yourself, the brewery tour is interesting.  If you are into History, you can tour the Missouri History museum that's in forest park as well.  Enjoy your stay.

Long time STL guy here. There is nothing family friendly in downtown at all. Forest park is ok and the zoo igreat but make sure you are out of there before dark. North and east St. Louis are worse than detroit. do not go to either. The science museum is loved by kids and adults alike. If you are into art of history those museums are quite good too. Oh, they are free also. Of course the mighest profile tourist attraction here is the arch. It is worth dropping by and checking out the museum underneath it. If you have no hint of claustrophobia then take the tram to the top of the arch. Sure it is a heck of a view and quite interesting but the most memorable part of the experience will be the ride up (heh heh heh, just ask anyone who has done it)
Since you will be at Barnes hospital you should know forest park is right across kingshighway. Unfortunately the warnings about forest park go for the area around the hospital.
A few random statements about the area.
DO NOT go more than a couple blocks north of the hospital, bad mojo there.
If any area you wande into seems bad it probably is.
The layout of the roads in the area make no sense at all. Get a local map and carry it with you whenever you drive anywhere.
It is hard to find good places to park downtown.
There are great places to eat in StL, check out some of the finest italian cuisine in "The Hill" area. (Oh yeah, you can wander around there after dark)
The Landing area is hit or miss but you with kids I see no reason for you to even bother to check it out.
Like to gamble? Loads of riverboat casinos all over.
The Zoo is great, so is the Arch.
Catholic? check out the Basilica.
Grants farm is great for animal lovers.
The tour at the A_B brewery would be a wet dream for most of the beer swilling alkies here. (Heck, it is nice even for more casual drinkers too)
StL for some unfathomable reason has done nothing at all with its riverfront. Nothing down there but warehouses and winos.
STL has a lot of gunshows, see if there is one around when you are here.
Keep away from St Louis Center, union station is ok.
Artistic? Laumeier sculpture park is interesting to those so inclined.
Downtown St. Charles and the antique area of Alton, Il. are really quaint for lovers of overpriced antique type junk. (seriously, I don't get it but antique lovers just gush about those places)
Hopefully this will help, any more questions feel free to ask.

ETA;
The city museum is privately owned and charges a fee though I am sure it is not very much. I have heard it is interesting but it is also full of rambunctious kiddies so if you enjot the screaming of a thousand ankle-biting hooligans then you will love it.

Gary(the obviously childless)M

It's a lovely shit-hole...

 I appreciate the replies

Do not take the G-19 to the Arch, or a knife. In St. Louis do not print or allow your weapon to be seen by the public. Lot's of sheep.      The Zoo is great and it is easy to spend a whole day there.

Good luck with your daughter.

I have two kids.  The City Museum and the Magic House are great to take kids to.  Both are privatley operated but cheap, and fun for grown ups too!  Like others said the Arch, Forest park,  Zoo,  Art Museum (All free) are great to see!  Enjoy St. Louis!!