My dog is about a third of the weight of the dog in question. He is
about 6 kg or 8 lbs. I bought a laundry basket to sit on the
non-traffic-side wide-loader. I use the free-loader clips to hook the
laundry basket into place through the basket's handle holes. I figured
the dog's weight would keep the basket from flipping out over bumps
(since the attachment was only at the top), but went ahead and added
another strap that went through one handle, around the basket and to the
wide loader. This basket could easily contain a much larger dog. The
only problem with my dog is that he's too small to get in by himself. I
have to lift him in and out.
My dog wears a harness. I fixed a strap with a clip on it to the top of
the non-traffic-side v-rack. I clip this strap (aka seat belt) to the
front of my dog's harness. I found that if I clip it to the back of his
harness (i.e. the usual spot), he could jump out (but this was with a
smaller basket). I usually clip his leash to the front of his harness,
too, since it helps keep him from pulling (or swings him back around to
me when he does pull).
Somewhere on the internet, I have seen a picture of someone carrying a
2-3-year-old nephew in a free-loader. The kid looked comfortable
enough. When I tried it, the kid wasn't that familiar with me or the
bike, so it was no-go.
Anyway, I find that the weight of my dog is negligible, but his basket
is on the opposite side of my bike's battery (I've got a stokemonkey).
A 2-3-year-old kid probably wouldn't be too much of a problem, either.
A lot of it has to do with experience. Heavier weights, like Tone's
80-lb dog, would require some adjustment in handling.
I have carried loads that were so heavy and so poorly balanced (this
last being the problem), that I had trouble getting started and needed a
push. After that, it was ok. Recently, I carried a large box full of
books that was heavy enough to make the bike wheelie when parked. The
box could have been 70-80 lbs. I could pull the bike down when I wanted
to ride it - so no wheelies in transit. I did have to give the bike a
serious lean to compensate for the weight. This lean for balance
becomes automatic with practice.