There is about a 5 hour drive from Dallas to San Antonio, so keep that in mind. Also, I-35 has several sections under construction, so your driving time may be increased.
What dates will you be in Dallas and what dates in San Antonio?
One of the best places in the metroplex to watch fireworks on July 4 is at the Texas Rangers game at Ameriquest Field in Arlington. They have a really good fireworks show after the game. Also, there are fun things for the kids to do in the ballpark if they get bored watching the game.
You might want to stay in Arlington while you are visiting Dallas as it is right between Dallas and Fort Worth, and there are plenty of decent hotels. Some of them include free shuttles to/from the Ranger games and Six Flages over Texas.
You probably won't want to do two days of Schlitterbahn. There is not that much to do there and the lines are pretty long. You'll probably be ready to go within 4 or 5 hours (or less).
In San Antonio you have Sea World and Six Flags Fiesta Texas. Sea World has a water park and a theme park area with roller coasters and other rides in addition to all the marine animal shows. Your family can easily make an enjoyable day there. Fiesta Texas is a massive theme park with lots of great rides and a great water park. It is probably a little more expensive than Schlitterbahn, but you will get a lot more enjoyment out of it. Also, if you go to Fiesta Texas, you might want to consider buying season passes for the whole family. The price for the season pass is basically about the same as paying for two one-day tickets, and it is good for entry all Six Flags parks in the country, which would include the ones in Ohio, Missouri, Illinois, Kentucky, New Jersey and all over the US. Plus, you can get the season pass at a reduced family rate by buying at one of the local grocery stores, credit unions, or banks.
Check out http://www.sixflags.com for more info.
you'll be passing through Austin on your way south from Dallas to San Antonio, so depending on your schedule, you might want to drop in on Lake Travis (beautiful scenery) or go see the Capitol downtown.
if you do make it out to Lake Travis, I would recommend having dinner at The Oasis. It's some beautiful sunset scenery, and you can bring the whole family!
http://www.oasis-austin.com/
I've been at college in Austin for the past four years, so if you'll be passing through and have any questions regarding the area, shoot me a FReepmail!