As others have said, all the Carnegie Museums (Science Ctr, Art Museum & Mus of Natural History) are excellent for kids and you could easily make a whole fun day just in them. The M. of Nat History has one of the largest collections of dinosaur bones in the world and it used to be possible to arrange to go in the back and see some of them being carefully chipped out of big slabs of rock by the staff archaeologists (it's worth a call to see if they still do that). If you check the Carnegie Library website closer to your date of arrival, you can also see what's being offered in the way of concerts and workshops for children and families at the library and in the museums.
You know about Kennywood Park (it has the oldest rollercoaster in the country and cool signs by some of the rides explaining the science of how they work), but two other, and I think slightly cheaper options are Sandcastle Waterpark and the small, old-fashioned Idlewilde Park (I think that's still around). In the old days, all the parks would give a discount to groups on all-day passes. It might be worth a try to see if they still do. You could probably work something out with the museums, too.
As for Pgh-specific stuff, in addition to the incline (the old hillside tram at Mount Washington which offers fantastic views), there's the PA Trolley Museum and the Sen. John Heinz History Center in the Strip District , which often has a sports-themed exhibit up. The Strip District in general, a stretch of road near downtown where all the produce and specialty food vendors are, is a neat "field trip" . I think kids would like seeing all the wierd stuff, -- giant fish being dissected at Wholeys, sausages and cheese hanging from rafters, and that kind of thing. While you're there you can also get a huge, cheap breakfast at DeLuca's or Jojos, or try one of the famous Pgh gh sandwiches with the french fries inside at Primanti Bros. The guy who runs the Strip's Enrico's Biscotti Co. is also super-nice, and would probably show your group how they make their amazing cookies if you call in advance.
We got the flora and fauna, too: the Zoo (which has a nice aquarium and some neat lectures, art projects, etc, mostly free, in the summer), the Aviary, and Phipps Coservatory, a big glass Victorian greenhouse in Oakland with a huge plant collection (this might be better for Moms, though, especially if you're gardeners).
And if you've got a number of teens and pre-teens on hand, they'd probably like East Carson Street in the South Side. It's the closest thing we have to a hipster haven in Pgh, -- a street full of cool, off-beat shops, cafes and galleries in charming old buildings.
Lastly, CitiParks arranges lots of free family events in the summer, so you should definitely check their website around May and see what's up for your weekend. My favorite of the CitiParks traditions are the free movie nights on Flagstaff Hill (just off the Carnegie-Mellon Univ. campus), which run all summer long. It's so pleasant to sit on that grassy slope watch a nice movie under the stars.
Wow, didn't mean to run on so long. Hope you have a wonderful visit!