This used to be common practice when flying with little ones. It was always a good idea to hang at the gate for as long as possible, allowing the kids to use the bathrooms, and move around to get as much excess energy out of their systems as possible.
Now? Everyone just HAS to be first to board!! Then, we see many kids getting restless very early in the flight! I understand the need when flying SW, but if you have seat assignments, then I'm not sure why the huge hurry to board with little kids.
So, yes, that's a terrific idea. You can board as soon as you are called, get the car seat installed and stow any carryon bags. Then, you dh can board later in the process with your DS. Great way to travel!