Laura,
With kids in the 4 to 6 age range, being near the Magic Kingdom is a great idea. You are going to want to do more in that park than any other, so staying close by is a great perk.
There are a few choices. You could go with two rooms at the Polynesian, Grand Floridian, or Contemporary, offering you monorail access to the Magic Kingdom. Of these any would do, though I think that the Polynesian or Grand Floridian make for a more family-friendly public space ambiance.
You could also consider still staying at the Deluxe level, but at a slightly lower room rate, by going to the Wilderness Lodge. That means a boat ride to the Magic Kingdom, as opposed to a monorail. The resort is one of my favorites, and it is a fun locale. This resort also has some bunk bed rooming options, which might appeal to some of the kids.
A third option is to look at some move varied accommodation options at the
Bay Lake Towers at the Contemporary or at the Wilderness Villas. Both of these Deluxe villas options offer you multi-room suites. For instance the two bedroom villas can sleep up to 9. The fact you get a kitchenette is a really great perk you can use for both together time and some in-room dining.
BOTTOM LINE: All of these options are great ones, but I think that the Villas options at either Bay Lake or the Wilderness Villas might be your best. The Bay Lake ones are more expensive, but have the monorail access. IF you can do that, great. But the Wilderness Villas are a great choice too!
Have a magical vacation!