I worked in a preschool for kids with some sort of developmental needs (meaning we had children who maybe for example were preemies or something and experiencing delays they will outgrow, learning disabilities, speech or language challenges but otherwise typical, all the way to children with obvious, lifelong disabilities like Down's or cerebral palsy.)
In the classroom I was in, I know both of us adults tried very hard not to say "good girl/boy". I know I did hear other adults using it and it bothered me. (I won't say we NEVER did it, but it was not a regular part of our vocab.)
We DID do rewards, and the first year I was there, it was a lot of candy. (not a *LOT* of candy at once, but like a Smartie or two or a jellybean was a pretty regular reward.)
At the end of that year though (which I came into the middle of) we did our classroom review, and we discovered (both of us being first year in the program) that one of our 'goals' was supposed to be to use as little praise as possible. (good girl/boy, good job, that kind of thing) (For anyone interested, this was High/Scope curriculum, it's what HeadStart uses. It has a book for classroom staff to go through and rate their classroom, we were required to do it a couple times a year.)
So....out of that came a goal to save our rewards for the 'big' things and rephrase our praise. (like commenting on what we see kids doing instead of "good job' or "I like" etc.) We also used 'alternative' rewards like stickers, special jobs, etc. to candy. (I remember one kid's 'reward'--after a time when I explained the word 'proud' was to hear me say that I was 'proud' of him for something. Still praise, I know, but this was a kid who was really struggling with some things and that was working for him, I think in part because he'd ask me if I was 'proud'--he had learned that new word and was working on applying it and taking 'pride' in himself for being able to apply that word to what he had just accomplished. I think it was almost more him thinking through this word and applying it to what he was feeling too...if that makes any sense.)
Now I'm soon to be on the other side of it, as a mom of a preschooler. I really don't know how I would handle talking to ds's preschool teacher about praise.
The bad boy/girl, I would have to say something---probably pointing out the difference between telling the child they need to do something differently and telling them they are a bad person. (I don't know if I'd go so far as to say this, but my brain would be saying that anyone who believes that a preschool age person is capable of being a 'bad' person already needs to seek other employment.)
I would (at least try to) do it in a way that doesn't come off totally condescending, but assumes that these people may not realize what they are actually saying to the children, and once they do pay attention to it and realize it, they will WANT to change the message they're sending out. (because they, like you, love your kids and want to do the best they can for them--they SHOULD if they're in the right job.)
The candy issue, I would just flat out say what you think is appropriate in terms of food rewards. For me, it would be that I prefer alternatives be used most of the time, and to please let me know when he has candy/sweets/junk in school so that we can talk about it at home and be learning about making healthy choices throughout the day. (like if you eat treats at a party at school, you should choose something healthy for snacking at home.)
(again I think a teacher who cares would change their reward system rather than single a child out--if you don't want your dd to have candy because of cavities, I think I'd change my rewards to not be candy and if a kid asked, I'd use it as an opportunity to talk to the whole class about candy and cavities.)
If the majority of snacks are things like cookies, sweet cereals, etc. I'd be talking to someone in charge--those things are out of the teacher's hands usually. I'd go to whoever it is in charge of buying and budgeting for food, with ideas on how to go fresher for cheap if you've got some.
