It's a crapshoot. You can have balmy temps or very humid weather (like the deep south). The closer to July you come, you run the greater risk. It's been unseasonably cool so far this spring without much humidity. Typically, mid June is pretty nice. I really wouldn't let the weather decide for you, though. The zoo, the botanical garden and City Museum are all great for kids even with the heat. The Boathouse in Forest Park (near the zoo) is also a great place to eat outside and rent a paddle boat.

Thanks for the info. The more research I do, the more interested I become. Seems like quite a few wonderful museums to see.

You are well informed southbound. And as nanecam said "it's a crapshoot."

Just make the most of it - you have wonderful plans. The zoo is wonderful - you might want to check out the children's museum downtown.

Having lived in St. Louis several times plus general area I do remember hot humid summers including June. Maybe "pretty nice" as nanecam says but you will appreciate airconditioning very likely. However, as mentioned do explore the wonderful zoo and in another location botanical garden.
Other sites too.

Bill in Boston

Funny, my husband has a business trip there mid-June as well, that I'm thinking of joining him for, for a long weekend midway. If you're a baseball fan, check the Cardinals schedule - he's discovered that we could go to a game against Kansas City on the weekend, AND our beloved Phillies will be there during the week! We've heard the botanical garden there is one of the very best.

Before the solstice is a lot likely to be nice than after.

Go to the zoo and Garden in the mornings, and air-conditioned museums after lunch. It'll be fun.

My Texan cousins who've visited botanical gardens around the world were very impressed with ours. Don't miss the Climatron and Temperate House. There's a kind of fun "Tree House" project there now. Under, not in the trees, though, darn it. I'm looking forward to an exhibit of architectural photos there starting in June.

If you decide to take in a Cardinals game in summer, try to get a seat in the shade if you can. I went to a day game here in July and made the mistake of getting a bleacher seat. It was scorching, 100+ degrees at full-on humidity -- and like many such folks, I retreated to shady parts of the ballpark after a few innings, preferring to stand in the shade while still able to see the field. Lots of other folks had the same idea.

Be prepared for all weather types - it may even be chilly! My birthday is mid June and I have
had to wear jackets at night some years!

Have fun in Saint Louis!!!

Ive been to Six Flags a few times in June and it was pretty warm.I would take a light jacket to be on the safe side.Have fun on your trip and be safe.

Thanks for the shade tip for the ball park. Turns out I can't join him for the long weekend but he will surely take in at least one game if not two. Also for the tip on Botanical Garden in the morning...he is most definitely not a morning person, but I'll pass on your wisdom and see if it convinces him when I'm not there to urge a little!

Anyone have any favorite barbeque place to send him?

Pappys is voted one of the best and has won many national awards. It is in the city.

http://pappyssmokehouse.com/

a local chain is called Bandana's - they are everywhere and also very good.