The fireworks are only when the park stays open until 10pm.
That's the other tradeoff when going at the busy times of the year. The park is packed, but they stay open until 10pm and you get to see fireworks.
You won't get to see fireworks on the 21st, but if you arrive the gate early each morning, the rides will have short lines and you can do the baby swap thing.
In the afternoon, it will be hot and you'll want to leave or go to a restaurant like Mythos and have a 2 hour lunch.
Some people don't like to leave the parks in the afternoon because it's a long walk to the garage and takes time. But I think it's a good plan to leave around noon, then come back around 5pm as the tired are leaving and the temps cool down.