Hi Melissa,
How exciting! Your first Disney trip...and your kids are at a wonderful age!
For our family of four, once our plane arrives, it takes a good three hours to get our luggage, take the transportation to our resort, get our room and get re-organized to head out to our next adventure (a theme park, etc). I suspect the same will hold true for you, plus you'll need to fit lunch in there too...that means it will be about 2:30 or 3pm before you are ready to head to a park.
I would start by checking out the theme park calendars to see the park hours, parades, fireworks and special events for your arrival day:
Magic Kingdom
Epcot
Disney's Hollywood Studios
Animal Kingdom
I can understand wanting to save the Magic Kingdom for another day. So I would suggest Disney's Hollywood Studios (where you may be able to see Fantasmic, check the park calendar for times). Pick up a 'Times' guide as you enter the park (or before you leave your resort), and work your visit around the show times for such wonderful events as 'Beauty and the Beast Live on Stage' and character greeting times. There is so much to see here...
click this link for more ideas!
Melissa, I hope this helps...have a wonderful family vacation!