"Remember your tall sitting position and heart to heart, watch your left hand thumb, tall 3 finger, p-finger over the sound hole, and start soft so you can play a crescendo!"
This is the laundry list of thoughts that I often have while hearing a student play, and I'm sure I've heard many parents say something similar to their young guitarist. As teachers and home-teachers (you parents), we can become overwhelmed keeping track of all the multitudes of minute details that happen in a polished performance: good sitting position, correct hand positions, clean nail tone, remembering left and right hand fingering, playing musically, keeping a steady tempo, planning ahead, and so much more! When we are teaching and everything seems to be falling apart, we tend to focus on all of those things that are falling apart. The result is that we the teachers are overwhelmed, the student is even more overwhelmed, and there is more negative to point out than positive to praise. No one is having fun learning in that scenario.
Instead, approach practice with a single focus point. Make this intention known before playing. It could be a point that was worked on in the lesson or it could be something that you've worked on for several weeks through review pieces. Make it as specific as possible, give a reminder right before practicing each piece every time, and give your feedback only on that one point. Make a game out of the focus point: the student gets a point every time she plays through with focus on the task at hand; the teacher gets a point anytime that focus point is neglected.
The hardest part of implementing this is putting aside everything that you see "falling apart" and keeping your focus and the student's focus on that one point. But by keeping the single focus, you are able to give more specific positive feedback. And remember that even those things that are "falling apart" probably just need to be made future focus points and will easily be resolved later.
The analogy in my mind is playing catch: can you manage to play catch better when you have one ball being thrown or when you're playing with eight balls? At some point, it becomes overwhelming and your game of catch becomes a game of dodgeball!