I'm the same way about baby talk.  I don't mind it so much if it's to a pet but its always kind of pissed me off when it was to a fellow human.  As a teenager working as a rehab assistent in a nursing home, I remember cringing at the way some of the nurses' aids - MOST of them - would baby talk our elderly patients.  They got OLD; not stupid for heaven's sake!  They deserve respect!  When I was raising my children and heard baby talk addressed at them, that bugged me too.  I don't believe babies are stupid.  I believe they have the souls of people that have been here before, full intelligence with only the memory fogged of who they were before.  Talking baby talk to them would, in any case, dampen their language relearning skills and cost them a certain amount of personal confidence.  So I always ever had one of two reactions to people baby talking my kids: Saying to them, "It's okay.  They're pretty smart.  You don't need to talk down to them." Or, if I were really irritated, I'd address my child in a normal conversational tone, asking them something or sending them away, and then address the baby talker in baby talk, making my point that way.  One thing I never did was put up with the baby talk. 

This is my opinion...I wouldn't say anything to them.. Talk to your daughter privately and explain why ppl do this. It's not worth embarrassing yourself by correcting well-meaning friends.. 

If your daughter is aware of it, I think you handled it perfectly.  Put the power in her hands to say "I am not a baby, please don't speak to me as though I am."  I think it's hard for people who don't have children to understand that it's not just your children you are embarrassing, but yourself when you talk to children like they're stupid.  If you correct them it may step on toes, but if your child does it, then I think they'd hopefully respect her for it.

I hate it when people do that. My grandma always says "pweeze" to my cousin instead of "please"because that's how she says it. Well she will never say it correctly now! I would just ignore everyone who talks like that. Obviously your daughter realizes it, so it's not going to hinder her language development. These are people you may never see again so just let it go.

I highly doubt anyone is purposely belittling your children. Many people will adjust the way they talk based on the skills shown by the child since not all kids are on the same level conversationally. I worked in a daycare for 2 years, and we had 3 year olds who could carry on a conversation and 6 year olds who were still struggling. Every child is different, and to talk to them all the exact same way doesn't always work. That being said, I feel it's your job as the parent to politely let others know your children don't need the baby talk. If your daughter is coming to you with her concerns and not addressing the offender directly, maybe she's not ready to handle that responsibility on her own yet? Ask your daughter if she'd be comfortable speaking up for herself, and if she is, great! If not, do it for her until she can do it on her own. Unfortunately, people are not necessarily going to change the way they talk to all children, they'll just use more adult vocabulary when addressing your's.

My husband and I don't use baby talk because we feel it's important for our children to learn the correct way to pronounce words. I actually cringe when I hear people using baby talk, especially when a child who is old enough to say words right doesn't because Mom continues to say them wrong. Despite all efforts though, some kids will not be advanced verbally or conversationally. My oldest, who is 6, didn't really start talking until she was 3. Our 3 year old is very smart and can carry on a conversation. Our 2 year old is not too far behind the other two, but he was a late bloomer like the oldest.

I wouldn't be offended at all if a little kid asked me to quit speaking to them like they were a baby, if they asked in the way that you suggested. I'd be pleasantly surprised that one so young could make her feelings known in such a polite and articulate way.
And I think people should learn from a young age how to stand up for themselves appropriately. That in itself would eliminate quite a few problems.

This is true for a variety of things, but when you meet new people, you have a baseline assumption about many different things about them, and react accordingly (whether you even realize it or not). As you talk with and get to know them, you adjust to the new information about them. So... when I assume a person is a thieving, bumbling idiot upon first meeting them, it's nothing personal. It's just my experience with the average human being. I'm in a mood today. Anyway.
While I wouldn't tell her, 'say this', I wouldn't really tell her she couldn't, either. Unless it's "piss the fuck off". lol If she gives you a "Mom, a little help here" type look, jump in.

First, take the anger out and give the teacher a call.  You don't want trouble for your husband, so this could be a good opportunity for your child to listen to you speaking very politely and pleasantly to another adult in problem solving.  Such as " My child (name) really likes to talk like adults do, as does her brother.  I have a few things here to tell you that the kids can do, so they will be able to shine and maybe not be embarrassed.  Then I'd find something really positive to say about the teacher, even if you are so pissed you can't stand it.  It will be a good way to start things off in a wonderful light, and your children could benefit.  Also, thirty six years ago when I was giving birth, I had your exact same feelings, and was offended by people talking down to them.  It did them no harm, and they weren't bothered by it because I hadn't talked about it to them, rather, just allowed them to intellectually grow and function at their own pace.  Now I find myself a grandmother of a 13 month old who can say around 15 words, and darn it all to pieces, I've caught myself saying stupid things like "Gwamma loves you."  You get the drift.  And regardless of what other family members are saying in baby talk, we still read to him, and he's still picking up words like his father, my sonm did.  So I think if you tone it down a little, because your kids are going to be super smart, they need not know of your anger with this.  Just give the teacher an extra friendly call, and I think you'll be surprised.  And let your kids know that this is  how we handle stress.

I don't have kids and am rarely around them, so when I talk to children, it's usually the opposite.  I talk to them like they are adults, and expect them to act a lot older than they are. 

As your daughter speaks with these people they will soon adjust their manner of speaking to her.  She simply needs to be encouraged to speak correctly and use her vocabulary properly and they will get the message.  Teenagers... no probably not, but adults will figure it out when she answers in a long well thought out reply instead of two words.  Not sure correcting them will do much more than make her appear to be a pretentious obnoxious child which nobody wants to be around.  She's much better off learning that she can't control how others behave, only herself.

Her self-esteem should never be based upon what other people think of her and how they treat her.  Encourage her to find her value in other things (in my book it's God-esteem, not self-esteem... how does God see her?).

@greene_lily@xanga - I was interested how this was going for you, I related to your story. I'm glad she is taking a stand for herself and that she has self-confidence. I also think little ones need their parents to visibly protect them sometimes, so FWIW, I applaud you for how you've handled this.

Tell the truth though, did it for a moment catch you off guard when your daughter chided you for teasing her?