a primary school kid's confidence

as [tuition] teachers, do we want to build up or tear down our students' confidence?

of course, "tear down" sounds quite brutal... but... let's say one tries to build up a kid's confidence... that's all fine and good until they start getting yaya-papaya and insisting that there's nothing wrong with their working when there're obvious mistakes.. case in point: i've had a kid doing 199 + 11 = 200 and insisting that it was correct...

but if the kid doesn't have confidence, then it becomes a problem of the kid not even attempting questions that he would be able to do... case in point: same kid as above... once he sees even 1 fraction on the question, he'll say "i dunno how to do"... forced him to do it and... guess what? he did it... whoopie...

both these incidents happened on the same session... wondering what i should do to him...