I'm new to these forums (been reading them for a couple months) but not to backpacking or parenting. I mentioned this thread to my wife–her reaction was something like "Sounds like hard work, but how cool." She's a labor and delivery nurse, and mother of our three kids (7 months, 8 years and 15 years), so knows a thing or two about both children and medicine. One of our kids needed some intervention at birth (minor but life-saving) so we know a bit about risk and the importance of modern medicine, as well.
Stupid and irresponsible? Two well-trained, experienced mountain guides have carefully planned a walk at reasonable elevations, along a well-travelled route lined with villages, cell-phone service and staffed "refuges" that offer beds and hot meals served family-style (it only took me a minute with Google to confirm huts along the G10 are nothing at all like the shelters on the AT, something Nick and his wife already knew of course, but which the negative posters here apparently didn't bother to learn).
In my reading, Nick has more than addressed all of James K's valid concerns, and been more than tolerant of the implied threat that such a trip would, in the US, be considered a criminal offense (how else would parents be explaining themselves to a judge, unless arrested by someone in law enforcement?)
The notion that 3 month old babies require "weekly/monthly" immunizations is just flat wrong. I am strongly pro-vaccine, and wouldn't do the polio-only route that Nick and his wife have chosen, but that has no bearing on their planned hike. If anything, the baby will be far less exposed to diseases in the mountains than in a dense urban area.
Bugs? I can understand this being on the radar screen of someone from the Southeastern US, and Art Standt's bee sting allergy example sounds compelling, until one realizes that "a 4 mile hike" could easily be covered in under an hour (given an emergency), so the allergic reaction argument is really saying, "Until your child is stung by a bee (and proves non-allergic), it is stupid and irresponsible to ever have them more than an hour from a hospital." Do you really believe that?
I would argue that most of the posters here who think Nick and his wife are being "stupid and irresponsible" are thinking of some hike in _their_ imagination, not the actual hike that is (presumably) being enjoyed as I post. People aren't even paying attention to the obvious–for example, warning about the dangers of untreated water for a baby that will be drinking nothing but breastmilk, assuming diapers will be washed in rivers, or suggesting a raft trip is less irresponsible because the baby will be able to crawl around during the day (3-5 month olds don't crawl). Art, have you ever carried a baby in a frontpack, backpack or sling while hiking? I have, my wife has, and the mothers posting here have as well (all of whom, curiously, seem to support the trip). Your concerns about comfort and rest are badly misplaced.
Finally, and admittedly a little off-topic, what *is* stupid and irresponsible is that the richest, most powerful nation on earth lets millions of its children go to bed every night malnourished, ill-clothed and ill-housed, badly educated and without any real access to health care. Since the Netherlands and every nation of the G10 route through the Pyrenees solved that huge child endangerment problem a long time ago, what a Dutch couple chose to do or not do with a single infant isn't at the top of my list of things to worry about.
I look forward to reading more about Nick, his family, and what is surely a wonderful adventure.