Mad Science - Bellaire (or at your home)
Who: Parties are geared towards 5-12-year-olds, but a Little Top Party is offered for the youngsters ages 3-4
When: Any time on weekdays and holidays (yes, open on holidays); times slots on the hour from 11 a.m.-6 p.m. on the weekends.  
What: 45-60 minutes of a Mad Scientist-led, interactive show complete with make-your-own slime; 45-60 minutes of pizza, cake, you name it. 
Extras: Party add-ons include cotton candy experiment, dry ice experiment, and/or build and blast a rocket.  Goody bags can be purchased and free downloadable invitations are available.  Want a party pic of all the kids looking like real scientists? Purchase white lab coats at the venue (birthday celebrant's coat included with party fee). 
Anything else? 15-minute set up time; no decorations allowed; outside food is only allowed during certain time slots and incurs a fee.  

Little Beakers -The Woodlands and Cypress
Who: There is no max number of kids (comfortably holds around 25, but parties as large as 40 have taken place here; no limits on age, but the typical age is between 4-10-years-old).  
When: Weekday evenings, Saturdays from 10-5:30 p.m., Sundays from 12:30-5:30 p.m.
What: One-hour Scientist-led session with four hands-on experiments, including take-home slime and snow, and an "explosion" for the birthday celebrant; one hour for general merriment
Extras: Last minute party planning? This place has you covered from the cake to set-up to goody bags.
Anything else? You can bring your own food or have the venue order pizza for you. Bonus points for a searchable date-availability calendar on their website.

Photo courtesy of Little Beakers 

Houston Museum of Natural Science - Museum District & Sugar Land
Who: 20 kids and 20 parents (additional charge for a larger room that holds up to 40 kids and 40 adults) 
When: Basic party times are 9-11 a.m., noon-2 p.m. and 3-5 p.m. daily. The Deluxe Party can be held at any two-and-a-half-hour timeframe during regular business hours.  
What: The museum really pulls out all of the stops. With a basic package, you chose from one of seven themes (some themes are recommended for older kids). The two-hour party includes a 30 minute craft, 30-45 minute tour or exhibit, and then 45 minutes for cake and confetti. A deluxe party includes everything from the basic package, plus exclusive access to the Dinosaur overlook and one add-on (see below). Invitations and thank you notes are also part of the Deluxe package.  
Extras: You think of it, and they offer it as an add-on. Magician, balloon artist, face painter, live animal presentation, bug presentation, and simulated space mission to name a few. Professional photography is even available.  
Anything else? Note that some of the themes are only available at the museum-district location. You get a 30-minute set-up.  

Club SciKids - Various Locations
Who: Children ages 4-13
When: Parties are one hour.  
What: Pick a theme, a science demonstration (elephant toothpaste anyone?), and a make-and-take experiment. Goody bags are included in the base price.  
Extras: Paper party supplies are available for $2 per guest. The birthday kid gets a shirt and science gift with the party.  

Idea Lab Kids - Various Locations  
Who: The lab offers themes based on age: 9 and up, 8 and under, and preschoolers.  
When: Saturdays and Sundays
What: Pick a theme (such as Jr. Scientist, Aliens vs Astronauts, Rockin Robots.  In addition to science parties, Idea Lab offers other party themes such as art parties and cooking classes) and then a package (Deluxe, Premier, or Ultra). Parties last two-and-a-half hours.    
Extras: You can bring outside food, and there are a variety of optional amenities, including a candy station, chocolate fountain, a photographer with a memory book option, a character visit, and karaoke.
Anything else? What else do you need with a chocolate fountain and karaoke?!

Top image courtesy of Mad Science