I think this is such a wonderful question! I have a lot of views, working with social workers and child psychologists as much as I do.
For the "good child" - especially girls - who, while quiet and calm and mellow are still able to say when they don't like something, don't want something, or aren't comfortable with something... I'd say in general you don't have much to worry about. Just keep nurturing their *judgement* - their ability to decide what works for them and work to achieve that. Keep an eye on how they deal with adversity - if they seem to prioritize pleasing others/not causing a commotion over looking out for their own needs and wants (within reason), then encourage them to speak up.
For the "good children" - again, especially girls - who are quiet, calm, mellow but also avoid confrontation, do NOT state clearly when they don't want, don't like, or aren't comfortable with something... then you should pay more careful attention and take more purposeful steps to nurture their "independent voice" or ability to establish and hold their own boundaries.
PPs noted that being labeled "the good girl" does instill this belief in a child that the thing about them the world most values is their ability to NOT make waves, not cause a stir, not even cause dissent. Sure, that is a great attribute, but the world is way too harsh a place for a child to believe that they need to present that more than presenting what is important and comfortable to them. You don't want your good child to later be a pushover, victim of bullying, or someone who can't advocate for themself when they know they are being taken advantage of. Sure, these things usually happen to everyone at some point - I'm not saying you can completely prevent it ever happening. But as a parent you CAN prepare your child as best you can to try to avoid those situations or to succeed and be safe in them if they're not avoidable.
A few things you can do:
1. Absolutely encourage your "good child" to speak up about their preferences. Once in awhile (if not regularly), ask what they want for lunch/dinner and make just what they want. That may seem small but especially in larger families, the quiet/good child's preferences are skipped over in favor of pleasing the loud noisy cranky kids (and adults) who - if you don't give them what they want - you'll hear about it all day. Make sure you are seeking and listening to your good child's voice on preferences for food, clothes, t.v. (if you watch), activities, etc.
2. Now and then encourage your child to talk about things that make them uncomfortable and ask them how they deal with it, and how they feel about how they deal with it. Ask them for situations or people they don't really like, ask why, and engage them on that. They need to learn it's ok to not like everyone and everything, and that it's even ok to talk about it. Good kids often feel like they can't say anything negative, even when it's really bugging them, and you don't want that. This can also help you learn more about how your child problem-solves and whether they feel satisfied with the results they get.
I know I'm using adult language to describe this, like "are you happy with the results you get", but I obviously wouldn't say that to a 2 yr old. With my 2 yr old I simply ask, after a visit with an aggressive playdate friend or a loud sound that scares her "are you ok? did you like visiting susie? Do you want to go back again?" and I listen to her (she almost always says yes anyway, and I think she truly enjoys it, but she has said no once in awhile and I listen to that).
3. And with older kids who can definitely express themselves, this is going to sound totally opposite of the advice we usually get, but encourage SENSIBLE risk-taking in your good kids. Ask them if there's something they'd really like to do but that they think they never could do, or you'd never let them do, or they're afraid to do. Engage both the crazy stuff (like ask them why they'd like to drive to California alone at age 12) in terms of maybe there's a story you could encourage them to write about it, or you could go see a road trip movie, something to say "Ok, you driving is completely out of the question, but let's have fun with this and do this instead"... And for the stuff that isn't so crazy (like "I want to go rock climbing" or "I want to tell Susie that she should give me my doll back" or "I want to be on American Idol") find ways to nurture/encourage your child to take little steps, either for fun or in all seriousness.
What's important about doing this is you are encouraging your good child, who maybe usually doesn't want to make waves and is fine with just accepting what comes to them, you're encouraging them to dream, to experience uncertainty and then hopefully success in something they didn't even dare talk about doing, you're showing them it's ok to try something really new and then fail, and you're also encouraging in them to think about themselves and what they like and teaching them it's ok (and even a positive thing) to step up with their own individual personality and dreams and preferences.
4. And in any situations where someone is mean or rude or tries to guilt your good kid, in my opinion it's best to both model for them how to handle it by addressing it on the spot (in whatever way you do that), or if you're the type of person who has trouble dealing with uncomfortable things at the moment they're happening, please (and do this with ALL kids!!) at least engage your child soon after (within 24 hrs) about how they felt, what they think would be the best way to handle the same situation if it happened again, and applaud your child for them thinking the situation through and verbalizing what they didn't like about it. It's so important, especially with "good kids" and especially especially with good girls, to try to normalize and nurture them taking care of their own needs - emotional, physical, psychological, intellectual - which includes helping them develop mechanisms for dealing with discomfort or bad stuff. It's never too young to start talking to them about it, even if they are 4 months old and have no idea exactly what you're saying, if they bump their head or get knocked over by a bigger kid, comforting them and acknowledging what they're feeling "Oooh, that hurt didn't it! I understand, you aren't happy about that! But you'll be ok." or whatever you say, but mainly that you are communicating 1) acknowlegement of their discomfort/pain, 2) that it's ok that they're reacting and crying/screaming/sad, 3) that you're there to comfort them and you love them even though they're saying "OWW I'M REALLY UNHAPPY RIGHT NOW!"; AND 4) if they're older, you're giving them a chance to express how they feel and when it's right, to problem solve the next time it happens again (how to avoid it or deal if they can't avoid it). That includes what they may want to say to a bully or who they want to go to for help or how they want to handle a situation.
