Comments like "my 4 year old could do better!" or "don't sub him in; he's terrible!" seem to be becoming more common coming from the stands during youth sport events.
If your child is playing in a team event, and some other kids' parents start criticizing their play, how do you respond?
This is tricky. On one hand, this is not pro sports. On the other, it IS sports. My own judgement would depend heavily on circumstances:
If this is an activity that the kids' parents paid for the kid to participate in, the kid has a right to participate, no matter skill level. If that's the case - point that out EXACTLY as is: "that kid has a place and a right as does anyone else who paid for the activity".
OTOH, if this is an amateur team that anyone gets to play in for free, you may object to the form, but to be honest I would say you'd be wrong to object to the spirit of the comment. In a sports team, the better players should get bigger chances. Furthermore, IMHO, showing the kids that they get to participate at the cost of edging out better players teaches them WAY wrong things in life ("I deserve this" instead of "I earn this").
Verbal, or even physical, abuse, aimed at the referees and coaches seems to be even more common?
That's simple. First offense, fore real abuse, you threaten to call the cops. Second offense, call the cops. Record on video to have proof. Ideally, rope in a couple of friendly adults to make sure the bully won't attack you for standing up (see also @Rory's answer re: "club together with other parents")
Also, every sports activity usually has a set of rules. Make sure to find them, verify if what those parents do is against the rules. If it isn't, work with the activity organizer to fix the rules. If it is in the rules, inform the offender.
What do you say to your kid?
This answer assumes the situation of simply stating "your kid sucks" and not actual abuse.
First, reassure your child that YOUR opinion of them doesn't hinge on their skill at sport.
Second, give them an honest assessment of their skill level.
Third, teach them that stuff like this ALWAYS happens in life, and they have only 2 good choices: ignore someone else's opinion - especially someone whose opinion really doesn't matter - AND/OR, win over the naysayer by training harder and getting GOOD.