Not parent, but I’ve been approached by homeless people plenty of times and most of the time they’re just ready to move onto the next mark if you decline to assist them, but occasionally they will accost you for your perceived lack of empathy. The situations can be tense and make you feel uncomfortable. So yes, bring approached by a homeless person would definitely ruin a commute.
These are quite minor and (IMO) verging on acceptable.
Try someone shouting in your face about how they're going to kill you and your family and that is closer to the experiences I've had with BART and homelessness.
