We're taking a family that's never been before as well (with kids). Hmmm how many days are you going to be there? I suggest MK first of course. I would put together a light and flexible itinerary. Do they like sit down meals? Are they ok with counter service? I suggest one sit down meal a day. That way you're not spending all your time lounging when you could be visiting sites/rides.
If you're going to be there for more than a few days I suggest breaking up the parks. Like visit Adventureland, Frontier Land and Liberty Square in the morning or until your finished. Then head back to your hotel for a swim or head to a resort or other park for dinner and fireworks?
The next day do the other side of the park: Fantasy Land, Toon Town, Tommorow land. Obviously just a suggestion. You can break it up any way you want but that way you're not trying to "do it all" in one day.
You WON'T be able to see it all so prepare them for that. Maybe disccuss before hand some favourites of yours, then divide things into "Must Dos", "Should Dos" and "Nice to Dos". Work your way down your list.
Have a Great Time!!!