I've been the last 3 years in a row and have two step kids that haven't come with us: they are 3 and 5. I'm not sure if we have them the weekend of the expo, but if we do I will not hesitate to bring them.
A younger child that cannot walk, I would recommend a wagon. The ground should be ok for some strollers, but last year was a muddy mess so I would recommend a heavy duty wagon like those seen at Home Depot and Lowes with pneumatic tires.
The sun can get intense, wind too. Pack sunscreen and perhaps lotion. Kids love snacks and juice too of course.
Lastly: there is so much to see I think anyone can have a good time. People are very friendly and relaxed: it's not like a home show or in town Expo where people try to lure you into entering a drawing or something like that around every corner and bug you about it. You can walk through the vendor areas (some of which is well shaded) and peek at products in a leisurely manner, check things out in a hands on manner, get a drink or bite to eat from a stand (I highly recommend Chuck Wagon Soda if they're here again this year...only time I drink pop all year) and just check out cool rigs from vendors and individuals alike.
I honestly think if your wife and daughter have any interest in the outdoors, they will have a good time going.
As for the route, Google shows I5 to I40 the whole way. A longer way, but very scenic in my opinion would be to drop down southbound on 89 in Ash Fork and then continue on 89A eastbound through Jerome, Clarkdale, and Sedona and then getting back "on track" in Flagstaff. You can even take a forest service road to the Expo from Munds Park just south of Flagstaff on I17, ad take the equally as gorgeous drive back to Flagstaff via Lake Mary Road.
Enough ranting..hope this helps!