Earlier quoted context omitted.
I agree that kids in an urban environment can be very rewarding. I disagree that the urban environment found in the Bay Area is conducive to kids. Visiting the local park is great. Visiting the local park when the walk is littered with human excrement and discarded needles, not so much. No parent in their right mind would ever go on the BART with a child.
Why wouldn't you take your child on BART? I'm not looking for an argument, I'm genuinely curious. I live in the UK but I've never visited the SF area.
I don’t know if you live in London, but I would say they compare to the Northern Line it terms of quality. But at the end of the day the real difference is about riders. I would say you do notice more issues like littering (including dangerous litter like needles) and robbery in the US train lines though.
However, just like most public train lines, you have to learn to mind your business, be aware and don’t lallygag. Also, stick to using it during the day.