I have to agree with everyone, 2 weeks is not enough time to drive pretty much anywhere in the states and still experience it and even worse with a 2 and 7 year old. 2 day road trips were bad enough as a kid.
Would the leaves be changing that early in New England? You could do New York, Mass, Vermont, New Hampshire and Maine in 2 weeks I'd say, or Atleast a large chunk of it. failing that, I would also recommend the North West. But I'd start in SF and go north (time restriction and not quite as hot). You wanted to do a cruise, so maybe drive to Seattle and do one of the cruises up to Vancouver or Alaska, although would $4k be enough for a cruise as well and there is the time issue there too?
To bring the cost down there is always camping, if you are adventurous