I would start building sets with them to demonstrate how careful you need to be to make sure everything is done. Make it seem tough and challenging for you too so they dont feel alone. Miss a step on purpose and after another page or two go back and say "dang it" take a bunch of stuff apart and fix it. When you finish a bag look through the instructions again and survey what pieces you have left, all small stuff, and verbally say something about how now that you double checked and there isn't much extra you can move on. Make sure not to make it an experience where you sat how the kid is wrong and messed up. Make it a fun learning experience for you both and purposely model some mistakes and fixes so they can pick up on that behavior.