Hey Minimate,
We're a family of 6 too! We have stayed at the Residence Inn SewWorld/International Center. As you might guess from the name, it's in Seaworld's "backyard." If you're splitting time btwn Universal & WDW, this place is a great choice. 2 bedroom/2 bath suites, 1/2 way between Universal & WDW, very short drive to either with light to no traffic (by back roads) and a complimentary hot breakfast included with your room.
If you are only going to WDW, I think there is also a Residence Inn in Lake Buena Vista...Possibly newer?... but I don't have any personal experience with that one. Good luck!