John,

I have crossed the US and Canada about a dozen times.
A couple of times alone, another few with my wife
and the rest with kids.

My short answer is if this is just a "fun-family-
go-get-the-van" trip then I think 2 weeks is enough.
Especially if you plan the trip around the kids.

If you were trying to make this THE trip across
the country with lots of sightseeing the I would
say 2 months would barely be enough.

Go for it, just plan according to your situation.

Serge