Personally, having traveled alone with two wee ones on a trip that still gives me the chills to think about (screaming baby an entire flight from Dallas to Phoenix, until the last ten minutes when he puked all over the aisle....) I would tell your parents you'd prefer to come the following Christmas. Or would a summer train/driving trip be easier? Are one-way car rentals prohibitively expensive? (where I live, they are VERY cheap certain times of the year if I'm driving to a west-coast destination where they're in demand.
An RV?
Can you tell me what company you're referring to? They're pretty expensive up here, but if there's a company that does cheap one-ways, maybe they've got a Canadian division that might do the same thing.
Thanks to everyone who answered the actual question of how to make the trip, and were objective and fair about their reasons about not travelling with infants - it's always interesting to hear other people's stories. I've been through a fair amount of airports and bus stations where I've observed parents with kids - including some really young ones - and the only stress
generally appears to be coming from the parents who haven't thoroughly prepared for the trip, although that's obviously not always the case - sometimes stuff just happens and there's no prepping for it. My personal experiences and observations have shown me that planning is key, and I've taken my son camping solo (when DH couldn't come due to other plans) starting as young as 11 months and we've had a great time every time. That's included some international trips, as well. Maybe I'm just weird, but because I consider childbirth to be the hardest thing I've ever had to do (so far) and survived it just fine, then everything else can be managed. I mean, the good moments have always been more frequent than the bad, and the ratio just gets better with planning and flexibility. We're pretty nomadic, and having kids hasn't stopped that - just altered the process a bit. :)
As some people seem convinced that all of my common sense and sense of responsibility as a mother has potentially gone out the window due to pressure from my selfish and obnoxious parents (who have never asked anything of us they knew we couldn't or wouldn't handle, and have no issue when we say no), and publicly condemned a complete stranger as a borderline negligent parent without asking for any other details, I suppose I could mention that the planning and preparation process in our household usually takes three to four times longer than the actual trip itself, and usually start weeks if not months in advance. (Notice the current timing.) We don't just pick up and go, and we have backed out of a few expensive pre-booked events that were not in the best interest of our son's welfare to attend, regardless of the lost cost and hurt feelings.
That would be just plain stupid with kids, and make for a miserable trip for everyone, including the people being visited. I watched a coworker who was so determined to go to the Caribbean that she took her 14-month old daughter on a 8-hr flight with a recurring and persistant earache, with the go-ahead from her doctor (so I wouldn't count on a doctor to squash travel plans.) I had to put up with the complaints for two weeks after she came back about how she had to spend practically the whole time in the hotel room with her poor child who was absolutely miserable. Duh - what did you expect? Buy travel insurance, cancel the trip, and stay home - that's the responsible (and sane) thing to do. That poor child; I can't even imagine putting a sick child through that.
After speaking with DH again, I discovered that he's gone from ambivalent to not really keen, which has squashed the trip for now, and I will say that the decision has absolutely nothing to do with the potential age of our newborn, for those who were concerned about that. Our doctor supports us 100% in travelling so long as the children are healthy - age is not an issue, whether two weeks old or not. She's not a big fan of staying isolated in the house to avoid germs. They're going to be exposed sooner or later, and with this new one's big brother being as healthy as he is (how many three year olds that have had three colds in three years even with being in daycare for two of them do you know?) she's really not concerned about illness in our family. Our position on vaccination is polar opposite to the ones expressed in an earlier comment, and she's fine with that too, so that's not an impediment for us either.
Anyways...for now the trip is off, simply because DH doesn't really want to go, and my parents don't really care when we come - they never did; going for Christmas was DH's quirk, not anybody else's. Maybe later next spring when we can take the camper and make a road-trip of it...