Mandatory for what activities? Does a kid in a trailer necessarily fall under that law?

My own non-scientific guess is at least to wait until the kid's neck muslces are strong enough to support his head. And if I did want to carry a younger child on a bike, I'd probably attach him to my body (front or back) and ride an adult trike supercarefully on an MUP. I saw a father doing something similar once, but considerably more dangerously: carrying his little kid (no older than six months) on one of those contraptions on his back, while he was riding a 10-speed. He was going pretty fast and doing some careless stuff for someone who had a load that precious on his back. I mean, normally I'd call that stuff reasonably safe (some switching between road and sidewalk, cutting through a parkng lot etc.), but not with a baby on your back! Though perhaps I was overestimating the dangers.