Mommybear, Hi and welcome to the Disboards! How fun to be planning your first trip!
Is your plan possible? Yes, I think it is! I think if that is what you really want to do, you should do it. I understand that you may want to get in as much as possible if you won't be able to go back to the WDW area for awhile.
I can say, however, that there is so very much to do within the walls of WDW that you will barely touch the surface with your plan. We first took our DS to WDW when he was 6. We are heading back in Jan for our 5th trip (DS is now 10), and we have not seen everything that WDW has to offer, nor have we stepped outside of its' confines yet. We plan to hit Universal and Discovery Cove, but have decided to wait a couple of years, when we feel our DS will get into the scarier rides at Universal.
You might want to consider 4-5 overnights at WL (been there and it is a great resort, BTW!), concentrating on the WDW parks you want to while staying on property,and then move over to one of the Universal resorts for 2-3 nights, such as the HRH, and then concentrate on the other parks.
Whatever you decide to do, I hope your first trip is a magical one!