Highly recommend the Dallas Zoo membership. We did this several years in a row when our kids were younger. Cheap and easy entertainment for the little ones. The membership also includes free parking.
Also, imo, the Dallas Zoo's new development has closed most of the gap between it and the FW Zoo. FW Zoo is still better, but only marginally now. Definitely not worth the extra drive from Dallas proper anymore in my book.
The DWA is nice, but I also think it's overpriced and relatively small. Not someplace we were interested in visiting more than once a year.
And whoever commented about worrying about being shot couldn't be more offbase. The chances of getting mugged in one of the open downtown lots is significantly higher than in the guarded, fenced-off parking lot of the zoo (which is a few hundred yards from the I-35 exit, btw). I spent 5 years working in the green BofA building and parked in a lot right by the DWA, and got robbed at gunpoint about 10 years ago.
[This message has been edited by DannyDuberstein (edited 9/4/2013 8:59a).]