Re: Bring a 100-400f/4.5-5.6L IS II or a 70-200f/4L IS to Paris?
Except for something like a safari where all that telephoto focal length might be useful, I wouldn't want to lug a massive 100-400 monster all of Europe with me. And while the 6D is a reasonable-sized camera for vacation, even the 24-70 would be overkill for me. The 70-200 f4 IS L is a marvelous lens, super sharp and contrasty, and a very reasonable size and weight. Far better for travel than the f2.8 version.
After years of lugging around heavy gear, my wife and I packed a lightweight APS-C kit for our last travel, and we loved it. Very few situations demanded anything else. The two SL1 bodies weight next to nothing. And the trio of EFS lenses we carried -- 10-18, 18-55, and 55-250, all STM versions -- covered an incredible full-frame equivalent focal range of 16-400mm, cost less than a $500 total (we paid about $200 for the superwide and only $109 for the telephoto, a refurb direct from Canon), and packs lightly and easily.
Were there times when I missed my full-frame bodies? Sure. But the APS-C cameras didn't leave me unable to do anything my full-frame bodies might have. In fact, the SL1 bodies actually gave me terrific autofocus for videography (something my 6D and 5DIII bodies were never able to do), and even a touchscreen. When editing, I saw little difference between the SL1 travel images from our last trip and the 5DIII/L-glass images from our trip to Greece and Rome last fall.
Most importantly, the smaller APS-C bodies didn't attract any THIEVES. It didn't hurt that our bodies are the white style, which seem to confound everyone who sees them. Big city thieves know their cameras and lenses, they know camera bags, and they work in teams to have tourists' gear far away long before they even know it's gone. It's been some years since I've traveled with a small child, but I imagine that children only create a greater distraction and make a well-equipped tourist and even more delicious-looking target.