I am not an instructor, but have seen some occasional behavior like you describe, and here is my opinion:
first, is there something in the dojo atmosphere that is fueling this attitude in those who 'can't'. i would expect there to be a few adults or children who are doing Aikido to help with self esteem, and who naturally think they can't...but if you are hearing it alot, they are obviously not getting what they need to work through that. In both a childrens class i helped with, and in a college club class with many beginners, i found that the best approach for someone who used those words was to point out something positive ('this is just one more step than the last technique, and you did that well' or 'it's all about balance, and you've got that part...' etc, and then encourage them to try as much as they felt comfortable....and then say something positive about THAT performance)...although i will admit some called me 'babying', i disagree. i would not do yonkyo on someone for two reasons: 1) pain does not help much to build up a person's confidence, 2) some of us do not feel yonkyo and would just look at you funny.
second, if a lot of folks are saying they can't, it definately points to problems beyond the students. if only one or two, then it should not be affecting the tone of the class/dojo/Sensei...WE determine how a person's words affect us, not the other person. if the sayings of one student seem to affect an entire class, then i would again look deeper.
just an opinion...