Okay, its winter of 2013, but hopefully this adventure is still on your agenda...
We homeschool our son, as well, and took 4 months to go from Boston to Seattle in 2011. Our son was 11 when we started but turned 12 during the trip. My words of advice are GO FOR IT!!! And possibly sooner rather than later. Though your youngest will be in better shape the longer you wait, you risk losing your oldest to other interests, etc.
I, too, went with a nervous wife. She rode across the US twice in her twenties, but it was a far different experience riding as a Mom. She worried before the trip and worried during the trip. (She's glad we did it after the trip.) My three points of advice for this:
1. We took the League of American Bicyclists Safety Course a few months before our trip. This made a huge difference. The course gave us all fantastic advise and practice for riding safely on the road. I'd been riding and touring for over 30 years before the course and even I learned a great deal. The course really helped improve our son's riding on streets and roads.http://www.bikeleague.org/programs/education/
2. When we started the trip, we always rode in this order: Me - front, Son - middle, Nervous Mom - back. We made it to the far side of NY State and Mom had a meltdown at a campground saying how nervous it made her feel watching her son on the road and worrying about his riding. For the next day, and the rest of the trip, we switched the order to: Me - back, Son - middle, Not-so-Nervous Mom - Front. This made a HUGE difference for her (and us).
3. We put all the appropriate safety gear on each of the bikes (as was reinforced at the safety class). Each bike had a flag, flashing front and rear lights, an ACA safety triangle, and a mirror. Cars gave us a wide berth 99% of the trip. Safety gear isn't a guarantee to be accident free, but if one of us had been hit by a car, we'd never be haunted by the thought of, "If only we'd been riding with _____________ safety gear, things might have turned out differently."
We wondered if our 11 year old would make it on his own (he was super head-strong about wanting to do the ride on his own), and were prepared to buy a tandem mid-trip, if necessary. He did start out pulling his own BOB trailer, but in WI we shipped that home/bought him small panniers/shifted much of his gear to my wife and my trailers. That would be one suggestion, to be the pack mule yourself and let your wife and kids have lighter loads. At least at the start. Another suggestion is start slow and then pick up. Our first week we rode less than 20 miles a day, second week less than 30 miles a day, and so on until the second month and beyond we averaged 40 - 60 miles a day. Once in a while we did 70+, but that was rare. We also took rest days to see fun things, stayed in hotels occasionally, and stayed with warmshower hosts (all of whom loved to see a family on tour, as most of their visitors are usually adults).http://www.warmshowers.org/
Oh, and read through the section of links I found on the web related to family touring: http://www.adventurecycling.org/forums/index.php?topic=6154.0
And get inspired by reading Joe Kurmaskie's books on his coast to coast adventures: Mud, Sweat and Gears
& Momentum is your Friendhttp://www.amazon.com/s/ref=nb_sb_ss_c_0_8?url=search-alias%3Dstripbooks&field-keywords=joe+kurmaskie&sprefix=joe+kurm%2Caps%2C310