Great post, kids are so literal with words we use, and slang makes it interesting :) I grew up always being corrected with my speech from my mother, and find myself doing the same with my kiddies.. And I never used baby talk. Actually my kids often surprise me with the 'big' words they use, and in context, but I have never curbed my speech with them, or dumbed it down so to speak..(I am an avid reader and wanna be writer though)
Using your example of speech needing to adapt with 'bless you', I myself stopped using that many years ago, when I stopped going to church :)
But we haven't gotten to one point and stopped evolving though, so no wonder our speech does as well :) These days we are Pregnant, as apposed to With Child :) another example...