Hi Min,
I think you would be wise to have them fully evaluated. When bright kids are not succeeding, there is almost always a reason beyond lack of motivation. Younger kids, especially crave the attention that comes with success and want to do well.
It's hard to say what is going on, but you mention a few things that might clue you in to general areas to look at. You mention that they have little situational awareness and do not learn simple household chores by watching. I am wondering if they have weak metacognitive skills - "thinking about thinking". This would be related to executive functioning. They could have some undetected ld - the testing should reveal this. You also mention that they have a strong factual cache of knowledge, but poor thinking skills. This could be related to executive functioning and also abstract reasoning skills.
No matter what, my gut says they are not doing poorly on purpose. Punishment or withholding privileges wouldn't be fair if there is some challenge in their learning profile that is a barrier to success. Their difficulties may not be an unwillingness to do work, but an inability.
Again, without knowing more about them, it is hard to say what is going on. But, it seems that you do need a full neuropsychological evaluation. I'm not sure how much help this is - only to reaffirm that getting the testing is the right thing to do.
